I don't really have any data for this, but I rarely go beyond +/- 2. Anything beyond that seems extreme. I look at my opponent's offensive distribution. If the majority of their points come from guards, I play (+). If the majority of their points come from bigs, I play (-). This works especially well against SIMs, as they don't adjust. And like sully said, if they shoot poorly from behind the arc, I'll play (-) and invite them to shoot more threes, even if more of their points come from guards. My cutoff for this is usually 33%.
Another factor is offensive IQ. If a given player shoots a lot but has a low IQ, I'll be more likely to let them shoot.