I like having 7-8 for JR on both of my interior players, but it's not a deal breaker. At the lower levels I would assume it's less of a problem. As you move up, it should generally be higher just because you're going to have more well-rounded PFs that can guard the outside shot more efficiently.
I had a friendly rivalry with another Irish manager a few seasons back- his PF was fantastic, on PR's National team and consistently top 5 in the MVP vote. His build was something like 14/4 JS/JR and while he'd routinely score 40+ on a lot of teams, my PF (9 OD) would usually hold him below 40% from the field. Obviously a small sample size (maybe 15 games or so), but experiences like this with the game engine forms all of our opinions, I suppose.