What may be considered optimal and what is actually practical are too different things. I don’t mind players stretching at the end of a session. Better than waiting till they are on their own when they won’t do it! Some players even like to stretch before training and i am yet to see it interfere with speed, power or strength. It would depend on many factors such as length of time stretch is held, tightness from other sessions in the week, potentiation from the warm up even if power was dampened etc.