I found that using a "square up ruler" is the key...I make 3,1/2" HST's all the time..my fav size to use. But also..I find that cutting the squares in the first place...just a tiny tad bigger...I line up the 3,1/2" line and then move it in that tiny tad just past the line..and then cut. I think that pressing the rotary cutter down pulls the fabric just that tiny tad of a bit, that makes a huge issue later on that you are having. Then...I draw the line, and sew..I use a seam guide as my pressure foot isn't exact..and then set seams and open and press...pressing hte seams first is a big help as it sets the threads flat and straight for opening...just think about it..those threads make loops which are round...and if they are not pressed flat...they can move and get crooked...again, just a tiny tad, but enough in quilting to make a difference. Then...use the square up ruler. and the points come out fine. Quilters have to remember ....a little here, a little there...means a WHOLE LOT later!