I think as long as when the spring is at its longest length the shackle still has an angle on it your ok. I have a trailer that was repaired and the shackle was at a point anytime it hit a bump the spring would lock up against the frame and I'd have to take a pry bar and snap it back down. a quick fix was to weld a stop on the frame not allowing the shackle to swing that far. i know that sound like WTH is he talking about but its hard to explain:flipoff2:My rear shackles are at that or even steeper. I wanted to tuck the frame side mount above the frame. It functionally works just fine. I don't get any odd handling or anything.
Shackles have an effect on spring rate too. I think the steeper the shackle angle the higher the effective spring rate.
In general I think you want the minimum angle at rest that allows the shackle to NOT cam over to full extension at the maximum droop of the suspension. IE you don't want the shackle to be able to get stuck on droop and try to go the other way accidentally.