Given a mast standing in column (perfectly vertical), the spreaders should be dead-on in line with it. Given backwards (normal) mast rake, the spreaders may be a little raked aft. The only time they're raked forward is when the mast is; and I've never seen that on a Western-style rig. More often they are raked aft to permit the upper shrouds to act as a 'helper' backstay (such as on the Bergstrom rig and on many boats with significantly-raked spars, such as the C44).
The spreaders may be 'encouraged' to have a little dihedral (upward angle from root to tip) when relaxed, but in reality, given a properly-tuned rig, they will naturally come to bisect the angle of the upper shrouds. They should never be 'forced' to assume an angle that is otherwise -- that'd mean a compromise of structural integrity (either something is too tight or too loose; and the natural tendency will be to correct itself, even if it means they break something in trying to return to where they should be).