Coins for the Super Mario Bros. series (and NSMB):
Your coin count carries over to the next level, so you would have to keep track of how many coins you have when the level starts. Then at the end of the level you'd do subtraction. It's not rocket science, but a few people have agreed that it's too prone to making mistakes when submitting.
Also, this is just my personal opinion, but: many of the levels, particularly in SMB1 and SMB2J, would be pretty easy to max out for coins except for the multi-coin blocks, which may or may not require perfect jump timing to max out. To me, it doesn't sound all that fun in theory, though to be fair I haven't tried it in practice.
Coins for the 3D Mario games (64, Sunshine, 64DS, Galaxy):
SM64 I'm not too sure about, because SM64 competition is covered well by sm64.com, and we want to focus on making charts which are not very well covered by other sites. SM64DS seems okay to add though. Sunshine and Galaxy are good, we just have to make a couple of decisions. For Sunshine, we should decide whether to allow/disallow infinite coin tricks. For Galaxy, we should decide whether to add charts per galaxy or per star.
