All the adjustments might have been made to assure the accuracy of the seam allowance but other factors may play into the finished seam. For example: The way you sit and feed the fabric under the foot may have an effect. I tend to feed the unsewn seam a bit to the left and have to constantly make sure that it is going in straight. I even mark a spot well in front of the foot to make sure I keep the fabric straight.

I have a bad habit when chain piecing. I look away from the seam I'm sewing to reach for the next piece and don't follow thru on a straight line. I have to remind myself to sew to the end of the seam then pick up the next piece. This caused my seams to be off at the ends of the pieces.

I have a 1/4 inch foot with the "knife" on the edge for my Pfaff machine. It helps a lot.

That straight line is necessary to keep me in toe! I noticed my seams changing when the line wore off. Especially when I get overtired. I think "It won't matter". It does matter...
Good thread.

I drew a line with a fine pt. permanent marker on the plastic bobbin cover of my machine. I could probably remove it if I wanted to but it does not rub off on my fabrics and 'extends' the line visually from the sewing foot downwards. It helps me be able to see the measurement better, especially when I'm sewing at night and the light is not the best.

I used masking tape to mark my 1/4in seam and it has helped immensely. (sp?).

Have used the masking tape with hemming, kinda figure that if all are the same, it's good.
I don't worry if more than 1/4 tend to hit 3/8 consistantly, can see it on the machine and have seen fabric fray more than 1/4 when washed :) Know this dosen't work with all patterns
Just my opinion :)

I use narrow electrical tape, red for the scant 1/4" and green for the center (for diagonals). These can be put from the needle plate down to the edge of the machine deck and if they go over the bobbin cover they can easily be cut thru at that point.

I also have trouble with chain piecing but am fine if only doing one seam. May be my short attention span??

I bought a quilting foot especially designed to gage your seam allowance while piecing. I haven't used it yet but I got it at Walmart for about 7dollars. That might help. I do hand piecing and picked up the wrong foot by mistake.But, I have it if I need it. lol