I would never train JS whatsoever. And I don't recommend it to anyone
I one season of JS training you get something like +4-5 JS, +2 HN & DR and some bonus (whether it's JS for guards or wingmen). If you train one on one for forwards, in one season, you get about +7 DR +4-5 HN +3 JS + 3 IS. I don't see the point of JS training