Mr. Floriani aside :lol:
I'd start with the muslin itself. If it is loosely woven, stretchy at all, very thin then I would use a fusible cutaway and plan on leaving it behind the entire piece to help stabilize the muslin. I admit, I keep a roll of Floriani Fusible No-Show mesh on hand :D
If the muslin is good quality (tightly woven, the same weight as the cotton fabrics and stable) then I'd start looking at the end use. If it is going in a project that will be washed often and much loved, I'd use a cutaway to help keep those stitches stabilized over the long haul. Otherwise, I think a tearaway would be just fine.
Just what I would do :wink: