I default to -2 on 3-2, and 0 to +1 on 2-3, depending both on how good my guards are, and how good my rebounding is. With slow guards, I lean toward plus positioning; weak rebounding forwards, I lean to 0. If both are true, I'm rebuilding, and I probably don't care.
On the center, it's a very important position to understand if you're playing 2-3. It's not like the center is in a one-on-one matchup. In a 2-3, the center's job is to obstruct shots, and rebound. To that end, his defense attribute rating isn't as important as his shot blocking and rebounding. I actually don't mind if my center's defense rating is a little lower than where I want my guard and forward averages to sit in a 2-3. Obviously, the higher the better, like anything. But a 65/95/55/95 guy (ATH, REB, DEF, BLK) is pretty well suited for 2-3, and will likely be able to hold his own defensively at the D1 level.