If it's for an exchange, NO tolerance for something LESS than the expected measurement. It's just not fair to the others who work hard to get the exact measurement. and it's NOT impossible to attain.
However, when it's for my own quilt, and I want/have to fudge a bit, I do it. We've all done that. If I don't square my blocks as I sew them, which is often the case when I assembly-line sew, I sometimes wind up with surprises. Then I have to decide if I can live with the differences in sizes. If I have extra fabric, I MAY make more blocks, so I don't have to fudge done to 1/8 seams. Not always possible though.