For every new project I check my seam allowance first. I hate doing it but
if I don't it will come and bite me. Take 3 strips of fabric 5" x 1.5", sew them
side by side, press and measure the middle strip. It should be 1". If not,
adjust your needle slightly to the left or to the right.
And for HST's I cut 1" bigger like Madquilter explained. I'm making lots of
HST's right now and sewing 8 at a time. Requires a bit of trimming but
comes out right.