I think any time of the day is okay to workout. It gives you flexibility and chance to actually feel like doing so. I think that's how it is to me. It may be more emotionally based upon rather than a time discipline. But it works if you really don't feel quite up to it and you still give yourself time to get on it.
I like to work out in the mornings so u can't let the task get away from me during the day. Like if I tell myself I'll work out at 5pm and then end up having a heavy dinner, I have to wait a few hours before working out and then it's time for bed. So I usually like to do it right in the morning, after I've had a big glass of water but before I've eaten my healthy breakfast.