I have a QB with 13 passing skill. But my top 4 receivers are only 8/6/6/6 in catching skills.

1) So is it worth still training the QB in passing still?
2) Or wait until the receivers have more higher skills?

Training him higher and adding higher wages at the end of the season seems kind crazy if the receivers cannot catch the passes and their training is far behind the QB.

The QB has hit his soft cap, so training may not result in a higher passing skill than current.

The receivers will add 1 catching skill point to catching every 3 weeks. So another 3 each before the end of this season. The one with 8 catching will only gain 50%.

Your QB has 1.5 Talent? Get a new QB.

To answer the actual question, it seems to be a combination of the QB passing + WR catching, so any point from the QB should help overall. With that said, if it won't make a difference in promoting, then you might want to take wages into account and train tacticals, or Fitness (I wouldn't have recommended that for a QB before, but with the new engine, it sounds like those will matter more).

Change promoting to relegating then.

If an extra point or two in passing is going to be the difference between staying in TD or demoting, believe me that the wage isn't going to be an issue. You'd lose more playing in D2 next season. I'd train the crap out of his passing except for that soft cap. Since it's capped, it's a tougher decision. I'd try to squeeze out a point or two and hope I got lucky, though.