It all depends on the sub unit. In some cases you can't trim down without chopping a point or messing up where seams are supposed to meet. In these cases I will ease in the excess or make a scanter 1/4" seam. But I will often trim a sub unit if I can. This is also why I probably like Deb Tucker's rulers so much. You make them oversize and trim them and the markings on them are such that you won't chop a point and also leave yourself a full 1/4" for the seam allowance. Whenever I make HST I always make them a wee bit oversized and trim.