How is this kind of decision making handled by the greater FGC anyway?
It's decided by the TO's on a case by case basis for each individual tournament. Of course, TO's don't want to piss off the community with rules that most of them don't like, so they usually do not deviate unless they have a good reason for it and most people don't disagree with it. The first couple of months up to EVO 2011, MvC3 was run BO3 except for winner, loser, grandfinals which were BO5. After that evo, the community basically came to an agreement that the game was too random to be decided on BO3, so more and more tournaments just started having UMvC3 tournaments at BO5 for the entire thing. It was a slow process though, as EVO 2012 only had every non-pools match for UMvC3 at BO5, and at EVO 2013 every single UMvC3 match was BO5 except for one set of pools. The reason that the tournament rules evolved like this was because as people got better, they got better at killing characters with TOD's (touch of death aka z2d). So matches went by faster and slowly the TO's found out that MvC3 tournaments could use 3/5 through more of the tournament and still run on time. Running on time is one of the most important issues to TO's.
SF4 top 8 at EVO 2012 was BO3 except for the 3 finals and went by really quickly as infiltration just bodied everyone easily, so to make SF4 not feel like it gets much less time than all other games and to give players a better chance to adapt, the entire top 8 at EVO 2013 was BO5. Southeast Asia Major 2013 ran two streams at the same time for multiple top 8 for games, so they had plenty of time and went nuts and made SF4 top 8 all best 3 out of 5, the winner and loser's finals best 5 out of 9, and the grand finals best 7 out of 13.
As far as I know, rounds in a game are probably the equivalent of stocks in smash. Except in games like MvC3, Injustice, or KOF XIII where the number of rounds can't be changed because it makes no sense, every game has never changed their number of rounds. SF4, SFxT, and MK9 have always been 2 out of 3. Soul Calibur and Tekken have always been best 3 out of 5.