I've made several commission t-shirt quilts as well. Adding a stabilizer to the shirts does add time and cost, but it really makes the quilt come together better.
Go to Google images and search for t-shirt quilts. As you scroll through all of the pictures, make note of design ideas that you like, and what you don't like. I prefer to use a sashing between every shirt so that your eyes have a resting place. I've also seen lots of t-shirt quilts that I absolutely loved, and they didn't use any sashing.
Before I start on a quilt, I number all of my shirts and put information about each shirt into an excel spreadsheet. This was in one of the books I bought on Amazon, and I use it all the time now. For each shirt I list the primary color, subject, minimum and maximum block width and height. One of the tips in this book also suggested that you only work in even numbers. Try to keep your shirt designs to 8", 10", 12" and 14". If you only have one design that's 8" wide, can add a complementary 2" frame around it to turn it into a 12" block. It will really make it easier for you to match up shirts with the same width.
Have fun making yours!