Thank you all for your advice. The customer's mother made the quilt top and had the fleece on hand. Ultimately, the customer said she would leave it up to me. I'll think about this one before I put it on the longarm since the opinions were mixed. Darn, I wanted y'all all to agree. haha I do really appreciate your opinions.

Your T-shirts already add a lot of weight with both the knit and stabilizer. I would only use the fleece (or batting and a cotton backing). I also think it could be a real challenge to quilt with knit, batting and fleece all wanted to move around.

I made a quilt with the fleece in place of the batting. It is a king size quilt and with the fleece inside, it it very heavy.

I often use a layer of white flannel instead of batting when I use a minkee/fleece backing. It might be my imagination but it seems to help avoid stretching of the fleece when quilting it.

I totally agree with this. Fleece backing doesn't need batting, and I think it would make a t-shirt quilt that has backing on the squares too heavy. Even for winter. lol

I use fleece on just about all of my t-shirt quilts and don't use batting. But I have never used a long arm for them, so that might make a difference.