The concept of smoking ribs meat side up or down has more to do with rib presentation and your method of wrapping (or lack there-of) than anything.
The first goal of smoking ribs is to set the rub and to develop color - simply for presentation purposes. On every smoker, the ribs should initially be meat side up.
If you're someone who likes to wrap ribs in aluminum foil with liquids, you'd place the ribs meat side down when you wrap. This way, the ribs are braised via the added liquids and the rendered fat/juices.
On every rack of ribs there is a meat side and a bone side. Put simply, the meat side has visible meat and the bone side has visible bones.
To help visually understand, we can look at a rack of St. Louis cut spare ribs.
The Bone Side
The Meat Side
The reason for smoking meat side up first has everything to do with presentation. Most folks eat with their eyes first and having the meat side up allows you to build better bark.
This is due to the the Maillard reaction (browning) of amino acids (from the protein) and reducing sugars as well as optimizing the interaction between the smoke and ribs for color.
Pictured below is a rack of ribs that's ready to be wrapped. We have an awesome deep red color and beautiful bark. With the bark setup, all we need to do now is reach tenderness.
In the case of pork ribs, the meat is both on top of the bone and intercostal (between the bone) meaning, there is no meat on the bone side to even eat.
Bone side down also means you won't have the potential for grill marks on your meat side - which would ruin your presentation.
When it comes time to wrap your ribs, most people will flip the ribs and smoke meat side down and bone side up.
The reason for this is because most people who wrap ribs will use tinfoil and they'll liquid wrap their ribs. The meat then lays in this liquid which braises the meat.
Note: Conversely, if you were to wrap and then place the wrap meat side up, the liquids in the wrap would accumulate at the bottom of the rib without doing much of anything for the ribs - apart from potentially steaming.
This is more pronounced on loin back ribs where the bones are curved. The liquid would accumulate under the curvature without interacting with the meat.
There are a variety of ingredients people will choose to liquid wrap with, usually it's a combination of:
Wrapping with the above ingredients is more so what I'd deem "competition" style as most of the folks who teach it are barbecue competitors. The goal in a competition is to pack a ton of flavor into a singular bite as apposed to eating an entire rack of ribs.
Personally, all I do is spritz a little bit of water on top of the aluminum foil and then lay the ribs meat side down.
Baby back ribs can also tend to benefit from butter simply because they're a lean cut of meat; Personally though, I don't bother.
When wrapped, ribs will continue to cook and render fat (lard). The result will be a build up of juices at the bottom of the wrap.
Essentially these liquids are braising the ribs (slow cooking in a liquid) which effectively speeds cook time while tenderizing the meat - rendering the collagen and connective tissue into a gelatin.
Aluminum foil combined with a liquid like water increases thermal conductivity which speeds cook time. This is because liquids have a much better heat carrying capacity than air.
The biggest reason for smoking with paper is to preserve the bark. Since butcher paper is permeable, it doesn't entirely prevent surface evaporation like foil which means your bark won't get soggy.
With that said, tightly wrapping with butcher paper still prevents surface evaporation or cooling - meaning it can speed cook time.
When I used to wrap pork ribs in butcher paper, I still preferred to have the meat side down.
Pictured below is a rack of baby backs that were butcher paper wrapped and smoked on my Pit Barrel cooker, meat side down.
Something that I think a lot of resources mislead people with is that while butcher paper is porous, it can still hold juices. I mean that's readily apparent in the photo below:
After testing lots of different methods, I still find aluminum foil to be best for ribs. Typically, I smoke ribs until they're 175-180F internal. At which point the bark is set, and all we need to do is continue rendering collagen.
Wrapping with foil for 30-45 minutes after the ribs have hit the above temperatures with only water in the foil wrap (and the rendered juices from the ribs), won't make your bark soggy.
Note: Even in side by side comparisons with how I smoke and wrap ribs, there isn't a ton of difference, if any. The butcher paper wrapped ribs do take longer though, so there's that.
Just to show how nuanced the world of barbecue is, there are also arguments in regards which side is the better way to slice - meat side up or down.
In my opinion, it's way easier to slice when the bones are up - just because you can actually see what you're doing.
This is especially true with something like spare ribs where the bones run at an angle and then transition at around the 5th/6th rib to be more parallel.
The reason people don't like to slice bone side up is because that means your meat side is on the cutting board which could potentially ruin the bark or remove some of the sauce and leave it on the cutting board.
To combat this, people like to pre-emptively sauce their cutting board and then lay the meat side on top.
The above is completely personal preference though. In my opinion, do yourself a favor and slice bone side up.
If you're someone who prefers everything that comes with "no wrap" ribs - typically smokier, darker, in some cases crunchier - then you should smoke the ribs meat side up the entire time.
With the idea that the meat side is your presentation side, there isn't much utility in flipping the ribs over to the bone side.
I know some folks like to flip no wrap ribs while smoking, simply to offer the membrane side some color and smoke flavor.
However, in my opinion, it's not super necessary, especially because people hold the bone and eat with the meat side up.
