Hi rh,
Very Good question. I too had similar queries coming into my mind at various times . Well IMHO the reason to wait till 3:15 is to be sure that the crossover is going to sustain till that time. For example, if the crossover happens now and near the end of the day NIFTY reverses and goes south then we will be in a dilemma. Also if this happens one time during the day it is ok, but sometimes it happens that NIFTY wants to have a dance between certain range, In such cases we will be coming in and out of the positions many a times. Which will add not only to the losses but also to the brokerage.
I hope I am not boring the members with such a long rhetoria, Should learn to concise the matter in a few lines.