Yes, I skip them unless they lead to a really good quest chain. You can make up the lost quest XP pretty easily by running an instance an extra time or three. I prefer quests vs instance grind but even classic doesn't need to be done to 100% completion just to level. There are benefits (rep, etc.) but they usually aren't worth the time, for me at least.

[edit] My comment applies to both classic and retail, though with retail's much lower XP requirements per level, it takes even less quest coverage.