Aim for the beginning of return window, this way you have more time if something comes up, always works for us.
That's not always possible, because you can't control the window start time. Our Everest and Safari fastpass return times actually overlapped even though we got the Safari FPs almost 2 hours later, and both overlapped with lunch. Trying to plan the order of attractions was hard enough as it was - now crisscrossing the huge parks even more is now necessary.
Fastpass should take stress out of a day at Disney, not add to it... Again, enforcing the window now when they never have just plain stinks. I will be providing this feedback to any Disney manager that will listen...