I also cut the front away from the rest of the shirt, unless I am going to want to use some of the backs when I need extra to fill in spaces. I also use any motifs printed on the back and sleeves. I never seem to be able to get all the blocks the same size as I am not willing to chop off parts of the design. I use the smaller pieces to fill in spaces in the quilt layout. If the motif requires me to use part of the neck ribbing, I just include it in the square. After stitching it to the sashing, it's fine as any other part of the shirt.
Also, if the design is vinyl or rubberized, the foot doesn't glide over smoothly so I tear small strips of waxed paper, perhaps 1" wide. These I use to cover the edge of the design so my sewing foot will glide on the paper and not stick to it. I don't actually sew thru the wax paper, just next to it. I don't usually use stabilizer but instead will use thin muslin or the tops of bed skirts to back my blocks.
I quilt around the designs before sewing them into the quilt and it's much easier doing one block at a time rather than a whole quilt. I use the backing fabric for the quilting layer, not batting.
Sometimes I add sashing as needed to each square to bring them all to one size. This is a quilt in which I used the sashing for that: [ATTACH=CONFIG]612475[/ATTACH]