I've done quite a few large counted cross stitch pictures. My mom is into it big time. I've lost count as to how many huge pictures she has done.
Anyways, she used to frame all of our pictures for us. She never used glass because it would hide the texture of the stitches and such. Well, after many many years, the pictures are I have are really starting to show the wear and dirt of just hanging on the walls. I wish now that she had used glass when she framed the pictures. My mom is getting up in her years now and doesn't do her own framing anymore. I think she actually has glass put in all her latest pictures.
If it were a choice I had to make now, I would definitely put glass over my pictures.