I think most of us struggle with these issues. I'm guilty of lots of no-no's in my quilting. I do use starch - sta-flo at a 50/50 mixture. Let it settle into the fabric before ironing/pressing. I do this to my yardage before cutting. I TRY to always use the same spot on my ruler but rarely do. A tip that I keep forgetting to use - put a small piece of painters tape or post-it on the spot on your ruler that you want to use for repetitive cuts. Will improve cutting accuracy. Most patterns seem to be designed for the elusive 'scant' 1/4 seam allowance. My machine has a setting for that so that part is easy. Otherwise - again with the painters tape or post-it notes.
I press every seam as I piece. I use steam. I press my seams open unless the construction process does not allow for this. I square up each unit as I piece as well. Sometimes I'm a little over; sometimes I'm a little under. I usually go with 'close counts' if it's not too far off. Fabric stretches and it can also be eased to fit together.
Certainly try to improve your accuracy. But unless it's really far off, don't make yourself crazy.