Originally Posted by
maviskw
I started quilting with clothes about 40 years ago.
The description you give is about piecing. The OP has asked about FMQing. I assume you hand quilted or tied those quilts? (Machine quilting wasn't 'in' 40 years ago, is why I make this assumption.)
Piecing with clothes doesn't really produce additional challenges. But quilting them does.
It seems like the OP really has 2 questions. For a large quilt for her mother- clothes are a great idea. But if she wants to get better at FMQing, sale fabric is the better idea.