It is true that cooler temperatures in early spring and fall are less stressful on new transplants.  That being said, you can still plant perennials even as temperatures get warmer.  The keys for success are adequate moisture and mulching.  While you may be able to skip watering for a short time with no ill effects on your plants during cooler periods, new transplants with less developed root systems would suffer during our prolonged summer heat spells - consistent watering , as needed, is key.   Combining that with proper mulching will help cool the roots and conserve soil moisture.  Picking an overcast day to do your transplanting or doing the work early in the day will also help reduce transplant shock.