I trained rebounding on my entire team for 2 weeks now.
I do this because I valua rebounding a lot, and I find that my guards should have decent skill as well, and they don't have it.
It obviously is more rewarding to train only the PF/C positions, since taller guys and/or guys with good inside skills will train faster, so the smaller guards which mostly have low insdie skills will train slow.
So unless you feel your guards need to improve in this skill (which is the case for me) I'd train either PF/C or SF/PF/C (if that is possible).
They are not your friends; they dispise you. I am the only one you can count on. Trust me.