seems to me that SIMs never set any of their players' 3pt frequency to -1 or -2 and so generally SIM coached teams are going to take 10-20 more 3 pointers per game. true?  

No, there are sim teams with very poor per ratings, and those teams shoot only about 15-20% of their shots from beyond the arc. 

Generally when I play a Sim team, I use a + defensive setting (coincidentally, yes, usually I do use a +2, but I may stretch out to a +3 or even +4, depending on just how many threes that particular team DOES shoot), but that is by no means set in stone.  I think you have to look at each team on a case-by-case basis to use the optimal defensive settings.  I agree that Sims in general shoot more threes than human coaches, but there are always exceptions.

Also, look at their 3 pt %- if they are shooting 25% behind the arc, don't you want them to take as many as they'd like?

Once in a while there is a sim team where the big men take way too many threes.  So I will play a minus defense to goad their bigs into taking even more threes.

I look at the 3pt% and their per ratings. If they take 18 3s a game and shoot 38% but have nobody over 75 per I'll play a minus defense. Odds are their % is on the high side because they've played some really bad teams. I rarely go + against sim teams, mainly because they usually don't have very good per ratings and my teams will generally have good defenders where I won't get hurt.