I've gone both ways; sometimes if some players are pretty close, I'll award a bonus for completeing the story arc to put them over (and those who weren't close just get the extra XP). Other times, such as today, had one player maybe 100 away, and another 37 XP away. They'll have to wait for next time, but I'll probably hand out some experience pretty early.
Another thing I've done, when players are close and I have a tough encountered planned, I'll allow everyone within ~500 XP level up. They don't gain any experience, but in every way they're treated as the new higher level. This includes experience calculations, so they'll end up getting less from the encounter- but, the encounter is a little easier.