On some older handpieced blocks, I've seen seams as narrow as less than 1/8 inch!
I've found this test to be very helpful and takes only a few moments:
Cut three strips of fabric five inches long x 2 inches wide.
Sew them together with your 1/4 inch seam.
Press.
Measure the unit you assembled.
It "should" measure 5 x 5 inches.
If it measures more - either take slightly deeper/wider seams OR cut slightly narrower strips
If it measures less than five inches - either take slightly narrower seams OR cut slightly wider strips.
(Although when I think of "seam width" - I usually think of the row of stitching which - on a straight stitch - is the width of the thread)