I, too, cut my squares 1" larger than the finished size I want. Draw the diagonal line, sew 1/4" on either side. I then trim placing the 45 degree marker on my ruler on the seam line and trim. It's added steps but, for me, ensures, accurate blocks.
I don't have any specialty rulers. I do have a 6.5 x 24; a 12.5 square and a couple of other smaller rulers but that's it. For a 8-10" HST block you may need a 12.5 square ruler to easily square HST's that large. But it would not be a bad investment either. Bloc-loc's; tri-recs; etc. I don't bother with. Just the basics. And try to stay within the same manufacturer of your other rulers.