Class promptly, on time.  Load up with as many vets and experts.  You want those two sailors to be the heaviest on your ship, they provide the most value.

For Sonar man, I believe the best strategy is valid for all nations: late-class it until Lv120.

Check out the following UK Sonar man. Potential base = 13; Repair = 11; Restore = 12. Classed at Lv120. Sonar range cap reached, while providing 4.9M repair true ability and 600 SD by this single sailor, plus less weight.



As for Planesman, you can class it on time since no air reload cap has been found yet. 

Seems there are two different explanation. Will there be more example for me to take a look?

He didn't grind it on a sub, the sonarman was levelled as support on a BB and classed to 2nd-1st-chf sonarman at 120.

And it only works because he made SDE very happy by putting 300 vets on it. If he did the same with  a more sensible 200 vets, it wouldn't cap sonar on a SS6.
Also it's a bit of a waste he went for 12 rest, 11 rep. UK (and MN) don't need to select for rest on their sub crew. With 4 engineer on board you'd already hit 900 SD.
A 13 pot, 12 rep sailor would have been better, even if it had a 8 base restore stat.

I give up arguing with purists who consider some mathematical ability to be the solution to some competitive threat.   Repair and restore ability are nice, but without any doubt in my mind, a trivial addition to the operation of a submarine.

Your focus with a sonarman should be sonar, not repair or restore.   Same with Planesman.

When you get the higher level ships, you can put a dedicated repairman on if you want.  I prefer all Chief Engineers because speed and OH are more important during battle than repair ability you may not use.  Besides, you're not going to go limp back home with 15 secs of air left and lick your wounds..you're going to go in to the battle, fighting to the end and sink as many ships as you can as long as you can survive.  That's the life of the submariner.