I have made numerous T-Shirt Quilts with out using interfacing. I used the Too Cool T-shirt method. It is really all about using standard sizes to cut your pieces. For example, in the quilts I made all the pieces were cut in increments of 4 inches (plus seam allowance). So, I cut 4 1/2 x 4/1/2, 4 1/2 x 8 1/2, 4 1/2 x 12 1/2, 4 1/2 by 16 1/2, then repeat for 8 1/2 , 12 1/2 and 16 1/2. The pieces are cut based on the size of the logo that you are cutting out. Then because you know that all pieces are in increments of 4 inches you know that if you put 2 of the smallest pieces (4 1/2 in. squares) together then they should match with an 8 1/2 inch square.
The only time I would use interfacing was if the shirt was really flimsy or it was a mesh shirt (with holes in it). I have never understood the overwhelming use of interfacing when making T-shirt quilts. The T-shirts weren't made using interfacing so why is interfacing needed when re-purposing them? As long as you cut pieces in sizes that should fit together, then you just use standard sewing techniques to 'ease' any difference.
This is an example of one I made. post #32
http://www.quiltingboard.com/main-f1...ml#post4790296