I tend to wear a lot of cosplays that deal with intense face/body make-up or prosthetics, I try to wear my cosplay shortly before I have to go into judging. I'm just trying to avoid getting anything smeared or rubbed off by extensively wearing it prior to the contest.
However, there have been times that I will wear something hours prior to the contest because it allows for the social aspect of cosplay that I enjoy.
Really, it depends on if you want to wear it out and I'm all for it or if you feel like you need to make it a surprise or fear that something might go wrong.
It's up to you! Just have fun with it!
Check me out on Facebook - EvilTwins Cosplay
Or here Adnarimifcation Cosplay
Next Convention: Katsucon
Seth Nightlord - Trinity Blood
Death Maiden - Diablo III
Wanderer - Journey