I also almost always piece the backs of my quilts. I like to use leftover fabric from the front both for added design interest as well as providing fabric for future repairs to the front if needed. T-shirt quilts can be thicker and heavier than normal quilts especially if you used stabilizer, so if you are really worried about it, why don't you make a small 'sample quilt' of a couple of squares of similar fabrics (front and back) with batting sandwiched in and do sample quilting stitching on it to see if you have any issues.