It depends. If it is a special pattern that may be hard to piece, I will cut the length of fabric. If it is a pattern that can easily be pieced, I cut across and sew the short pieces together.
What you are saying is that when you cut the length off (and you need to do that first) then you need to be careful that you have enough left over for the piecing of the blocks? Did I understand that right? You have to plan how much you need for piecing and how much you will lose due to the border strip being cut off. It's that math thing again.