I guess I'm in the minority that I don't measure my sides to get my borders, I go with "as-built" conditions. Up to that point every subunit of each block has been trimmed to size and should be consistent other than a thread width here or there. My width is determined and then like most everything I do, I put the borders on large and trim down. I pin the heck out of things, and I use my ironing board to keep everything smooth while I'm pinning.
Unless it is for show, it really doesn't matter to me most of the time if one side is an inch or so different, the only time I notice it is when I am piecing (ok and when I'm folding) and I need to make the units come out correctly. But for a plain fabric border? The tape measurement is not my friend.