How do you define a "one-shot"? In my eyes its a single nights gaming session, which means the likelihood of leveling up is non-existent. If the standard 13 encounters are used define what is necessary to level up, its unlikely that a one-shot will carry through all 13 encounters. This all depends on how long each combat or encounter takes.
The 1-shot games I've done in the past, were ones that lasted around 5-6 hours over an afternoon or evening.
With respect to leveling up, the players wanted to reach level 3 where they get to use another new encounter power. For such a 1-shot game, I was willing to be accommodating and leveling them up relatively quickly. (I wasn't following the guidelines that closely for leveling). Another option would have been to start everyone at level 3, but the players didn't want to do that for some reason.