Like some of the others here, I've been quilting a long time. With that said, I still goof on the HST's.
My biggest downfall is that I tend to IRON instead of PRESS.
Yes, I know how it's supposed to be done, but my brain still wants to IRON. The best tool I've found to help ME is the StripStick (
http://www.thestripstick.com/ ). Yes, this does press the seams open, and some ppl will scream about that, but as I said, this is what works for me. IMHO, the only time the open seams really matter is if you plan to SITD (Stitch In The Ditch, for you newbies). SITD truly requires both seams pressed to one side.
As others have said, I also make my HST's "oversized" and then trim them; I especially ignore anything that says "7/8", "5/8" and so on. I always up-size to the next 1/4 size - 1/4, 1/2, or 3/4. You can ALWAYS trim down... adding on after the fact is impossible (too bad the same can't be said for my body, it needs to quit adding on!!
)
Don't beat yourself up over this small of an imperfection. One of the "quilters' sayings" that I like is "If it's not noticeable from the back of a galloping horse, don't worry about it"... or something along those lines.
Only another quilter is really going to notice your imperfections - and then it's usually to commiserate and say "I mess that up too" or "Here's how I got better at this".
Good luck, we will want to see your finished quilt!!