I'll probably stop by this coming week....
as far as offset goes, pretty much whatever you need to:
1. clear your steering and brakes
2. get you the width you want
2a. fit on the trailer-- close corrollary to 2.
The main issue with offset is clearance usually, but bearing abuse is really good to keep in mind as well. Obviously, Rockwells under a tube chassis aren't going to be at risk. So I'd consider the other issues....
Now, PERSONALLY I like the look of a tucked-in wheel with lots of backspacing on Rockwells, and if you've got the room, go for it. Also helps keep the width down. I am not sure if you mean "backspacing" (distance from WMS to edge of rim), or "offset" (space from outside edge of rim to lug nut mounting surface).