It really depends. generally early morning or later in the evening.
This is if your time restraints allow for controlled shooting.
However, if you are on a tight schedule, like most projects these days, then eventually you will have to shoot in less than desirable light.
So if that is the case then you can hope for a few things:
1-its cloudy
2-the scene calls for the hot-high-noon look. (book of eli?)
3-you have no females you need to make look good in the scene
4-you can shoot the wides and silk the close-ups
5-you can block the scene under a tree or some other cover
While it is preferred and ideal to be able to shoot when you want, the reality is that it very rarely happens so you gotta adapt.
hope this helps
Todd McMullen
Flip Flop Films
Austin
http://www.toddmcmullen.com