|I don't add SA to patterns because I like having stitching lines (read below for my insanity reasons :)) I trace patterns onto fabric and then cut around the traced lines with rotary cutter. I don't try to cut SA uniform. I, then, put the pieces through unthreaded serger guiding the edge of serger foot along traces stitching lines. This trims SA evenly. You could add SA to patterns traced onto the Pattern Ease or other similar fabric like tracing medium if you prefer not having stitching lines on your fabric.
I like having stitch lines traced to my garment pieces. It makes it easier to alter a pattern, and sew more accurately where it really shows (matching collar poins for example).
It makes it easier to fit a garment and helps to see final adjustement I need to make to the pattern for future use.
I change SA width according to seam ,fabric and finish. I vary SA width within the same seam sometimes (crotch seam for example).