I think it depends - peak feeding time is right after sunrise and right before sunset.
But - if you've got a dog and depending on what they're feeding - it can be more productive not too push them until after peak feeding.
As an example, my favorite areas for grouse hunting are those with lots of clover - new pipelines, trails, etc.
If you hit those while the birds are out on the trail and feeding, the birds can see you coming from fifty yards and tend to bust out early.
If you wait till 9 or 10, once the birds have been laying down scent trails into the bush next to the trail, they sit tighter and provide better opportunities.
It also depends on your birds - most of ours seem to roost on the ground during the day and use spruce trees at night. But some areas the birds hit the trees during the day.
It's all kind of academic for me though, cause most of the time I just hunt grouse when I can, or during the middle of the day when the deer aren't moving as much.