I do like to play during busier days/times but that is mostly to allow more selection of tables and dealers. If only a couple of tables available and there are slow dealers then it's definitely far from optimal (especially if hand shuffle and poor penetration). However, at most bigger places I think the sharper surveillance people are on duty during the busy times. They are not there just to look for counters but look for anything that could be lowering the hold percentage for the house.

Optimal conditions are good rules, good penetration, fast dealer, automatic shuffler (not continuous), playing heads up and low enough minimum bet appropriate for bankroll. Very hard to get all of them at the same time but once in awhile it can happen.