First light is the best time to be able to set up with some certainty of where they will be and where they will head for, but I agree with most of you that all day long is good. The advantage to first light is once you have them roosted and know where they head after they come out of the roost, it's pretty simple to set up somewhere in between. But this doesn't mean you'll score. I don't know how many times I've roosted them the night before, set up between the roost and the ground feeding/traveling areas, have them come out of the trees just like you planned, have them coming in just like you planned, only to have them skirt around me.
The fun, and sometimes frustrating, thing about hunting turkeys is that they are very unpredictable. Just when you think you have them completely figured out, they fool you completely. There are so many variables involved such as weather, breeding patterns, whether the hens are already on the nests or not, etc. I've been fooled more than not when hunting these crazy things.
I consider myself a decent caller, but from my experience, a wild hen will out-lure someone calling any day. If they come out of the roost and right away you hear wild hens with them, it's very difficult to call them away. You might as well focus on a different bird. Although sometimes a dominant Tom stays with those hens and some of the Jakes come to you. If you aren't interested in a Jake, you may be screwed at that point. Like I said, they're often unpredictable, which makes it fun.
From about 10 am on, they are often still active, but they can come from anywhere. I've been surprised many times in mid-morning when you least expect it.