For me, it depends on the purpose of the quilt and how much the seams are "off". If it is an important gift quilt, I rip and redo to get it as perfect as I can. If it is just a practice quilt, one I am making for fun or a charity quilt, I give a little more leeway and only rip it out if it really bugs me. As others have said, lots of imperfections just don't show that much once it is quilted and washed. I don't expect perfection from myself.