Trapping for screen printing is generally determined by the substrate. Simple vector or line art that is to be printed with spot colors on a light colored garment usually looks best if butt registered. The inks absorption into the garment will create just enough spread to yield the cleanest looking trap but may be beyond the registration capabilities of your printer. Printing vector art on darker colored shirts requires an underprint. Techniques for underprinting vary widely, so always ask those relevant questions before imaging film.