all skills are good for all players but the question you have to answer is - for every extra pop in rebounding you get is it worth the extra skills you could have had elsewhere, bearing in mind the change in salary that results?
so you could spend 2 or 3 weeks training rebounding per pop and increasing this players final salary by 30k (or whatever it may be depeding on how high you take it) by the time he is 25 or 26 years old and finished training.
Or you could spend these weeks on other skills - perhaps more useful - and pay a lower salary for the rest of his career.
The efficiency of salary and skill mix also determines his transfer value if you ever want to sell him on.
In addition to this you should consider how important rebounding will be on this guard for your team - if you like to play faster offenses that results in a lot of shots then rebounding in general is more valuable. If you are weaker at rebounding at SF/PF/C than the teams you play against then a couple of rebounding pops would be more valuable as a guard.
ultimately the efficient training for this guy is to become an SG or a PG not an SF because of his height. so my advice would be not to spend any time training rebounding on this particular trainee. Focus on ID, then IS this season and see where you get to. If you can get them both to 9, or 9 and a bit, then id be tempted to leave these alone and wait for the eventual cross training pops to push them to a low 10 over the next few seasons.
His guard skills will need quite a bit of work so unless you fall way short of your 10/10 IS/ID goal it is probably a good thing to go to guard training from next season onwards.
have you any idea what kind of salary you want to pay for him in the long run?
Whether you want a PG or an SG out of him? This will help determine a target skill set to aim for that you can finalise in his last season of training.