When we had the freedom to choose when to visit (when we could take the kids out of school), we used crowd calendars to help select the best week to go.
I've never relied on them to choose which-park-on-which-day, however. The fact is, weekends will generally be busier, mid-week will generally be less crowded, and some parks will always be busier than others. Just choose the parks that suit you best each day (also keeping in mind park hours, special events, etc.) and go in with either a good touring plan, or a solid understanding of which lines are going to build the quickest. Smart touring will win out over a little bump in crowd levels at a particular park, every time.