I would suggest coming up with a sashing width the widest dimension you need to make all of your blocks line up nicely. If you need to shave a smidge off each sashing width, it likely is not going to be noticeable. If your blocks are say, supposed to measure up at 12.5" square and some vary between 12.25 and 12.75 (which I doubt as that is a lot and I'm sure yours are not off by that much) start with a sashing width of perhaps 3.5". If you need to go down to 3" wide, vary where you are putting those really off-size blocks (if possible) so you don't have a really wide sashing on one side or the other (or all that the top/bottom) so any variation of the sashing width is less visible overall.
Just my suggestion. I'm sure others will be along with other/better solutions. Good luck.