I disagree. There's too much variation game to game. I adjust the volume constantly.
MAME has a volume arg that makes adjusting volume for MAME games pointless.
Set your volume to be correct for the most quiet game.
For the louder games, just setup your frontend to run with something like "mame.exe sf2 -volume -12," which will run it with -12dB attenuation. Problem solved. Every game is normalized, and you never have to adjust the volume.
Problems might come up with butt sucking programs that don't let you adjust the volume, but I think almost all of them do. Most native PC games have a volume setting, and most standalone emulators do, too. It can be set up perfectly.