Another idea if you haven't been squaring up is to make the last rows you add 1/2" wider than the others you've been using. Then you can square up after the final pressing. There may be small differences and they may end up slightly wider/narrower than the finished strips on the inside, but it doesn't show up much on the overall design.
I use this method when doing a log cabin with several women sewing. Takes care of the little differences in our sewing.
Love the log cabin and have made lots of them.