not being Mrs. Fauxster, but if using water based stains it shouldn't (I haven't used oil-based in quite some time). For really complicated patterns I have also used contact paper instead of painters tape to prevent the darker areas from penetrating my lighter areas, (I did a star and moon pattern once on a dark, nearly royal blue/black background). It was much more reliable than multiple seams/overlaps/gaps of potential leak-through seepage of the dark stain onto my intended lighter areas.
Of course the poly coating has to be compatible with the stain that you use (i.e. water based on water based, oil based on oil based. Its usually best to stick with the same manufacturer of both and abide by their recommendations.