When you are making HST using the method of drawing a line and stitching and then cutting and trimming you have to be careful at two steps. Remember when you are drawing your line from corner to corner you are running along the bias grain of the fabric which is stretchy. Use a light touch when making your line. I like a fixon pen here because I don't have to press very hard to make the line therefore minimizing the tugging on the fabric.
The second place you have to be careful about is when you stitch 1/4" from that line. Here again you are stitching on the bias. Don't tug on your fabric or pull it through the machine. Let your machine gently move it along to minimize stretching. I stitch a skinny 1/4" away and I sew one direction going one way and the other direction going the other way.
If you are using the method of adding 1" to your square square beyond the finished size of your HST you won't have much to trim off when you square it up. If you are generous in your 1/4" stitching you may not have enough to trim. If this is the case then make your initial squares larger. You'll have more to trim. It doesn't make much difference how much larger your squares are beyond the 1" as long as when you trim you have the marking on the ruler going along the seam line. I use the Quilt in a Day where you only trim two sides then open it up and the square ruler where you trim four sides after you open it up. It just depends on which ruler I find first or remember I have. Both methods work great for me. My avatar quilt is almost entirely made up of HST.