Well I'm typically more of a true crown dip (paint) guy, but I've done plenty of crown staining in the past. I never worried anything about stain bleed or masking to produce a well defined line...simply because all my arrows get a painted crest and it covers the transition between crown and lower shaft. In many cases I wouldn't even bring the two stain (or paint and stain) colors together...basically leave an area of natural light-colored wood for cresting over. Doing that tends to result in easier crest coverage and yields brighter crest colors.