Dio, did you square your top before you added borders? That always helps. Square the top then measure for your borders by measuring through the center of your quilt for your border measurements. When you add your borders, your quilt should be nice and straight and square. (Provided you cut your borders accurately). Unless, I was entering the quilt in a competition I wouldn't go through the trouble of blocking. With experience you will gain more accuracy with your seaming, but the perfectly square top is rare. I can tolerate being off a 1/4 in. and it is not noticeable.
ETA: I square each unit as I build a block, then square the block. That adds to the overall accuracy of the quilt top. IMHO not enough attention or information on squaring is given to new quilters. My DH is handy with construction and in the beginning I learned a lot of squaring techniques from him. He recently taught me how to fit a beautiful quilting motif on a border when I was tearing my hair out in frustration :shock: