Since you are making them oversize and trimming down, just be sure that your seams are centered in the corners of your HST's before you trim. Also be sure your seams are pressed so that they lock together when you get ready to sew them in pairs and also when the pairs are joined to make the pinwheels.
One other suggestion is to sew your first pair from the outer edge to the pinwheel center and butt the next pair up starting with the pinwheel center and sewing to the outer edge. Leave the two sets attached. When you line them up to sew the final seam, the thread will keep the center together as if they were pinned.
I hope this all makes sense! I'm sure your will get lots of other helpful hints as well.
Good luck!