When I square up rectangle blocks, I look at the block as a whole, then decide where it makes sense to cut. I don't want to cut off any points, or any important parts of the design. I, also, don't want it wonky. Sometimes, all you can do is add an extra border to the block and upsize it so that it can be squared. It's all about making those design decisions when they are necessary. If you do add the extra border on the block, then make sure you recalculate any other sashing or borders that go in the quilt. I always keep in mind....There are no quilt police in my house and it is my quilt. I can make it into whatever I want!