I buy sheets from the thrift stores if they have a label it's great. But a person can almost tell by the feel of the sheet.
I personally like them for backings

I do not no why they think sheets to sleep on! Guess I aws not clear in my post.
I really want to know for quilting?

Yes! I do not think flannel sheets give a thread count, but I think most are low. I would really like to use flannel for backing my quilts.

I've used 180 count sheets for backing. Also, love to use good quality flannel sheets as backing.

Are there any guidelines to the good quality flannel sheets that you buy? I was wondering I guess what you mean good quality? There are so many kinds and vary in price. I am afaid the cheaper ones from Walmart might start to pill.

I misunderstood your original question. I thought you were talking about sheets for sleeping :)
I used to buy flannel sheets from LL Bean that were wonderfully made. It's been awhile though, but their sheets don't wear out or pill. I still have some.