We usually go in less crowded times, and this time we went when crowds were getting heavier (last week of May--just back). Historically, this week is still fairly light, but this year is off compared to most years. I would definetly stick to less crowded times. Not to mention avoiding the heat!!
I love the end of September, beginning of October. Light on crowds, still warm, but not HOT. I highly recommend that time of year. Next year we are going in December, which I'm looking forward to greatly. It seems like such a long wait from this past trip (19 months) that I may try and talk my DH into a long weekend in February to celebrate our son's 5th birthday there.
All in all, I've been in March, April, May, Setember, October, and November.