Yes I use old light colored sheets as well. Works great.

I use muslin for foundation on my string blocks and it works great.

use something really thin you have at home, sometimes people give me fabric and I use the thin stuff for foundation, I will cut them into different size of block just to get it out of my way

Best of luck on your string quilt. I hope to see a picture of it when you are done!

What About Muslin?
 

I had a really bad experience using inexpensive muslin. I used it in a quilt top that had around 150 4" squares. After the top was done, I got to thinking about the muslin. Where did I purchase it? What was the quality of it? So I took a large piece of the muslin and washed it in my washing machine as I would wash a completed quilt. Ye gods! It shrunk by about 20% and wrinkled so extensively that I couldn't even iron the wrinkles out. So I've been taking the quilt top apart and replacing the "bad" squares with better muslin. It is taking me forever. I should have known that you get what you paid for. Muslin isn't expensive, so it's only slightly more expensive to use higher quality muslin.

Well, I am not a Wal Mart fan but they do have several grades of Muslin and at a reasonable price. Also, different widiths.