i'd say morning or evening. It just depends on where you are I guess. About 1-2 hours before the sun goes down is usually a good time. Or in the morning before the sun is at its highest. just depends on where you are and how the sun is there.
I totally agree--morning or early evening if it's a full-sun day. If it's a partly cloudy day then you can use more of the day. Of course if you have a great shady spot to use then full sun outside of the shady area would just give you better light in their eyes I think.