If the work needed to prep on site is not too much... Go for it !! Have your ingredients pre-prepped, pre-portioned etc so all you have to do is combine and heat. Anything more may detract from your other categories unless it's the day before the meats.
With that said, I've seen quite a bit of storebought or pre-cooked at home stuff getting turned in by others. It really sucks. I think I'm done with desserts.. not worth the cash.