You should square the quilt by measuring across top, middle and bottom then average the dimensions and that is the measurement to cut borders. Measure both length and width. Then when attaching the borders, fold quilt and border fabric, mark centers then fold and mark , and fold again and then you can match border points and quilt points. When sewing this distributes the border fabric along the quilt edge and works the extra in over the whole edge of the quilt and gives you a square quilt to add the second and third borders. You can then cut subsequent borders at the measured widths and lengths and you will have a completely square quilt.

I wouldn't worry about 1/4" - - but if you have a thin boarder I can see where it would be a little more visible. Can you make up the 1/4" with the binding on that side?