Prbably depends on you. Since for the most part no one can check, you can sort of do what you want as long as you're having fun, but it is against the spirit of a game jam to start early. Though often people come in with an idea they have been brewing for days, weeks, months, or sometimes even years. I also tend to do some amount of preproduction before Jams, but some would call that cheating. Though once you take out prizes of any kind, what difference does it really make?
TL;DR: It's not judged, just do whatever will make it fun.
