Well, technically there is. It's best at night since there aren't that much people online so the risk of getting banned is smaller. Avoid weekends since there's more people online. But in reality - who cares?
you can get banned even the minute you start the bot its all possible there is no best time you take your own risk buddy
There's always a chance. You could argue it's just as dangerous at night because you're more noticeable running around than 5 or 6 others running around nearby.
I actually take the opposite approach -- at least with some profiles. With questing, I tend to go when there are lots of people online during the day. You end up blending in better than if you're the only one out there and you get stuck or something.