I think that depends on the competition in your league. If most of the other big men in the league are at 15/16, I would try to get your big men up, because they tend to see the majority of rebounds (although not always true- check out GM-Josef Ka's team when his ubercenter play pg- 20+ boards from the PG position!) Having said that, I wouldn't play a SF that had less than 8 RB and I like my guards to have at least 5.
I guess I would try to get my big guys up first,though.
Once I scored a basket that still makes me laugh.