Zhaleskra
Adventurer
I was really speaking more generally -- D&D's pretty much the only game I've looked at that does XP on a per-monster or per-encounter basis; most use a much looser per-session method.
D&D of course comes down to group preference, though personally I've never played in a group that didn't save XP awards until the end of the session.
While that is the default assumption for many games, there is no reason it cannot be changed. For example, I gave XP/skill points (really the same thing) after a single scene in a World Tree session. Not the assumption of the game, but I feel it alleviates some of the "it takes too long to advance in skill systems" sentiment. Basically, don't assume that because the game's default assumption is X, that every GM running that game is going to do X.
I doubt you'd get any complaints about giving XP after a scene, unless a player was really insistent on playing with RAW. Also D&D lost the per monster/per treasure thing at 3e, unless they put it back in 4, which I don't know because I only have one product for that edition.