I'd cut the plaid on the bias for sashing -- adds interest, and you don't have to worry about cutting it exactly right.
Bias can be a nightmare, though, unless starch it heavily first, before cutting. The way I do yardage is mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" it on using a large wall-painting brush until the fabric is saturated, toss in the dryer, and then iron with steam.
I have done bias binding this way, and it stabilizes the fabric sufficiently so you don't have to worry about stretching it while handling.
I posted a tip also about how to make sure your blocks line up when you do sashing without cornerstones. After you have your first row of blocks finished with sashing in-between blocks and on the bottom of the row, turn the strip over. With a ruler and pencil, mark all the seams on the wrong side of the bottom sashing strip. Basically you are extending the block seams through the sashing. When attaching the next row of blocks, line up the marks with the seams in the 2nd row exactly as if the marks were seams. Once pinned, if one fabric is slightly longer than the other, ease it in. With this method, when you are done, all the blocks will be lined up with one another. Otherwise you risk some wonky-looking strips when you are done!