I've always waited for the temps. to increase to maybe over 80 degrees, on the advice of my dealer. We have pretty cold tap water (50 - 55 degrees, brrrrr), maybe the chemicals dissolve into warmer water better?
Our Alk is really high, so it takes a bit of PH down to get it back in line, and we add it slowly over a couple of days. But we still use the tub as soon as the water heats up.