Friday early evening (6pm ish...after cooks meeting) would probably be the best time. Any earlier and you'll find most teams are still busy trimming and prepping. Any later and teams are getting ready to start their fires and it'll be too dark. However, earlier would be okay too as long as teams don't feel like your trying to shig their techniques. LOL
Anytime on Saturday after turn-ins is great too. Besides they'll probably have public tastings by half the teams when the gates open (usually before turn-ins even start). This is a great chance to meet some great people and taste some great BBQ.