We went in mid June last year, and there was a heat warning with temps of 90-100 degrees plus. How much the heat will affect you depends on what you plan to do while there and how well you tolerate the heat.
We opted not to do any hiking due to the heat (plus, we were tired because we were on day 21 of a cross-country journey). I think we would have explored more if it weren't so hot, but there were plenty of people there who were still out and about. If you are planning to just walk along the rim and check out the scenery, you'll be fine. If you're planning to hike miles and miles, you'll definitely want to consider how well you handle heat and make plans to hike safely.