You can do it now while they're warm if you want it really doesn't make a difference because the thermostat should get it to the correct temp. But if I were you I would get it all plugged in the way it should be with substrate and everything in the tubs and monitor temps for a few days before putting the snakes in the tubs (assuming they're not in there already). That way you can make sure everything is perfect before the snakes go in the tubs. Wouldn't want someone getting burned