I have no idea how you would use a fusible for HSTs, but think that you may be able to tell that sewing with foundation papers on the bias of the fabric minimized distortion when you are sewing any triangles. In fact, any time that you can stitch on the bias BEFORE the seam allowance is trimmed, your fabric will be more stable than stitching a bias seam with only 1/4" seam allowance beside the stitching line.
If you are not using foundation papers, I highly recommend starching your fabric before you cut your squares (or triangles) to size. After the fabric is dry and pressed flat, then cut your shapes and stitch.
And it's always important to be aware of exactly where your seam needs to be sewn. If you aren't sure that you are stitching an accurate 1/4" seam allowance you might want to check my tutorial here: http://good-times.webshots.com/album/569415726bHNKHF
You might also be interested in my pinwheel tute from an earlier BOM here: http://entertainment.webshots.com/album/570305305PZwEyD
Or...you can always plan to make your pieces larger than the finished size and trim them down after the seam is sewn
It's Not What You Gather, But What You Scatter
That Tells What Kind Of Life You Have Lived !