I usually set mine to somewhere around 1-2pm, except on sundays I will set it before 12pm even though I don't buy turnips anymore. I just like to see joan wandering around
If I'm playing late at night, I usually set it to early afternoon/evening. I'm usually up to get turnips from Joan, but I haven't bought any in a while. Too much in my boxes.
Harvest towns I set the clock just before 12pm. Check the turnip prices for the morning and than afternoon.
Also check the Sunday turnip price. If Joan sells for 90 bells I leave it on Sunday. Check my real towns and my harvest towns for high prices.
I don't TT much anymore because I have no reason to, but I usually play in normal time. If it's late (11pm or later) then I will change the time to 2pm - 5pm.
I TT to either 1 P.M or 3 P.M. I guess that way, if I leave my game open to do something else, a few hours later and I can still go to shops and my villagers will be awake.