Is your top flat when you lay it out on the floor? Measure all your sides and see if the top and bottom are the same and the right and left sides match their measurement. If everything looks good, then you can proceed. Only square your top if it won't cut off any of the corners from the blocks edges.
There is lots of advice on how to measure and put on borders. Smooth the top out but don't stretch it. I measure my sides and if they are within a 1/4 inch, I put a chalk mark on the border fabric piece at the measurement from the shorter side. I then add 1/2 inch to the border piece and cut 2 strips. I mark the other border piece at the same length and pin the border to the top making sure the top edge ends at the chalk mark. I ease in any fullness by putting the top on the bottom by the feed dogs. This usually results in my hitting both chalk marks and I cut off the extra fabric and do the top and bottom borders the same way.
The reason you want to get a consistent border is too much border fabric will cause wavy borders. Is it the end of the world if you get wavy borders? No, just try to do better the next time.