Yeah, we've done this for as long as I can remember too. Not sure who came up with it in my group, though I'm pretty sure it wasn't me. What we say is, "I'm y XP from next level. Can I take the deficit?"If they are close I let them level on credit. Go ahead and level is you want, but just remember you "owe" a few XP for the next level. Saves me from the old, 'Can I go on a quick orc hunt?"
Most of my players don't care that much about it, but my sons are always anxious to level.