When I sew and cut squares on the diagonal, I always have to trim each and every HST. And invariably, some aren't big enough to trim.
I'm using the scantest 1/4" I can possibly manage - which is actually 3/16". HST's have been my nemesis from the start.
I use the Triangulations CD - print out the finished size of HST or QST that I want, starch two fabrics facing, pin to the light side of the pressed pair, sew and cut apart. I don't even have to press with the iron if I don't want to - finger pressing folds them flat enough that I can piece them.
I do have to take the paper off the back, but I don't have to trim. They're right on the money for size and the seam crosses exactly from point to point on the square. :) (This perfect perfectness does not apparently enable me to set it accurately into a block - but at least the parts are accurate for those occasions when a block comes together correctly despite all my efforts to mess it up. ;) )
There are free patterns on the web that you can print out and try this method. If you like it, Triangulations costs only $22 at Connecting Threads - that's the best price I've found, anyway.
Here are a bunch of the free patterns:
http://www.quiltingandwhatnot.ca/Hal...-Triangle.html