Leveling up at the same time is fine and a simple way to do things, the only reason I dont usually do it is b/c it doesn't take into account XP costs of item creation/spells, or Bonus XP someone may get for certain circumstances/roleplaying, and it doesn't account for XP penalty due to uneven levels. Or for characters who miss an encounter, and yet learn just as much as the others.
That's not saying I think it'll be a problem, though.. I personally prefer the simpler route in PBP, and usually these factors don't matter. (When's the last time you crafted an item AFTER the game started? I haven't done it in almost a year... never online..)
OH, and PS - I'm Canadian, so I do understand Metric, but I didn't think Greyhawk had gone Metric, and even if it has, Jonathon obviously hasn't.