Two questions, really.
* Is it OK to have angled springs? Yes, manufacturers do it regularly -- just know how it affects vehicle dynamics for brake dive, wheel travel during suspension flex, etc. I favor a slight nose-up attitude on the springs, myself.
* Is it OK to use shims to correct caster? Yes, but if you use significant angle, be sure to use opposing wedged shims on the opposite side of the leaf to keep the U-bolts perpendicular to the springs (better clamping). Better still if you run the center pin through the whole wedge/spring/wedge setup. Watch out for pinion angle, tho - the absolute best option is to rotate the knuckles.

Randii