the sf-position is tricky.
'core' for me are both outside- and inside d, jumpshot and rebounding. i disagree with steve_karren here. the reboundskill has nothing to do with outside or inside focus in my opinion since is mostly a defense-skill.
offensively i'm with HeadPaperPusher. depending on your tactics you should choose to ad either some range or some insideshot.
a sf with a balanced skillset will help you to switch offensive focus BUT if your opponent is right with his tactical choices your balanced sf could have a hard time to win his matchup versus a specialized guy.
Rebound is both an offensive and a defensive skill.Rather, I would say that rebound is more important in offensive field that in defensive field,as it gives you a lot of second opportunities shots,while in defensive field you will not rebound if you not defend before ;D
I never said that rebound is not important for a SF,but you have to exploit at the most your training time to build the best SF possible,so you have to do your choices when you train a SF
A Sf has to have at least one good-developed shot,and both the defences to be able to play in the SF spot.So,whatever is your choice,you had to train Id and Od and IS or JS-Jr together.So,rebound is at least behind some skills.
For the inside Sfs,rebound is the 3rd skill behind Is and ID,at the same level of OD.For the outside SFs is at least behind JS-JR-OD and ID.
A versatile player(not forcedly balanced) need also the other attack at good level,so for an outside skill I would put Is at least at the same level of rebound,and for an outside player JS at the same level of rebound
So,It's not a question of"how much important is rebound",but is a question"how much I can remove from the other skills training to give him rebound?".And you often have to face the potential cap