I've made several t-shirt quilts, but I always use a stabilizer to keep the blocks from stretching out of shape. Since I cut the shirts based on the size of the design on each one as opposed to cutting them all the same size, I use extra blocks to fill empty spaces between my t-shirt blocks. I wouldn't have any problem using the back side of a shirt if I needed another filler block. I would treat it with a stabilizer just like the main parts.