Not sure if anyone can answer, but what is the difference in a single rating point in each category? In the engine, how much of a difference does it make between say having an 85 or 86 rating in rebs or per. Reason I ask is because by looking at ratings, the difference between 88 and 83 does not look large on the surface, but how significant is a 5 point difference?

You're right, no one can answer that. A couple points is not terribly significant. But also keep in mind that most categories are influenced by more than one rating (i.e. rebounding isn't just determined by reb rating and outside shooting isn't just determined by pe rating), plus external factors such as teammates, opponents, types of defense, etc.

But looked at in a vaccuum, no, a couple points is not very significant.

dalter, how about this: for rebounding I would assume that a reb rating point would be more importanat than an ath rating point, but by how much?

There's just no way to know that. You just need to use your experience and what you're seeing on a nightly/season-to-season basis to draw your own conclusions on that sort of thing. It's that kind of evaluation that makes the game tick.