On average, after the initial 5-day period, the crowdsale price will be exactly equal to the market price. On average, for an appropriate definition of average.
You're thus pretty safe contributing at the beginning of the period. On average, you're perfectly safe. At then end of the period it's a little asymmetric - you'll either know for sure that it's a bad price or you'll be uncertain whether it's a good price or not. Thus, you have more information at the end than at the beginning and this should mean you're better off waiting till the end.