If you get the erection you can use it, yes. Adviseable I wouldn't say there is an adviseable time if you dont want to incorporte it as an exercise (girth), so I think every time is good
Time makes no difference, many men wake with erections and it is a perfect time to take advantage and make some gains. When Ballooning try to incorporate doe Slow Squash Jelqs into the mix, they really step up the game.