I've made several projects from shirts such as you mention. No interfacing. The only shirt I've had trouble with had a lot of rayon in it and it frays terribly. I loved the color so was simply careful when I handled that one until I could get it worked into the foundation and went with it. Most of what I've done are wall hangings so will not be "put to the test" but I made a queen size quilt for my own bed and so far, even with use, it's all together! If you are working with knit shirts, you may need the interfacing but my experience with traditional shirts--go for it. (You are going to be amazed at how much fabric those shirts provide!)
I probably should mention that not all of the shirts I used are 100% cotton. Many of them were a blend. No problem--just that one that had a lot of rayon in it.
Last edited by illinois; 10-28-2018 at 02:41 AM.