I always thought the window decals come out nice and clean with no additional work to them. You just have to rub your finger around the edges to make sure there are no air bubbles.
If for some reason yours are not peeling off nice and clean try scoring around the window decal with a razor blade (after painting) before peeling it off to get a perfect line.
Maxx - FLM, MMM, CD Med.36x706s
Raja - FLM Ext. MM, 1512, 4s
SC8ight - MMM, 1520, 6s
D8T - MMM 1717 6s