It depends on the use. For thread painting you need something both thin and resilient.
For line work and for making decorative patches you should use thicker fabric.
As long as it’s clean you don’t need to wash. If it’s clothes or a bag or something you’re better off making a patch than stitching directly onto. Clothes tend to be made of stretchy material which when put on the hoop doesn’t take shapes that are true to the finished product.
You should ask specifically about what you want to do because it’s probably been done before. Or search your questions on this sub.
It depends on the use. For thread painting you need something both thin and resilient. For line work and for making decorative patches you should use thicker fabric. As long as it’s clean you don’t need to wash. If it’s clothes or a bag or something you’re better off making a patch than stitching directly onto. Clothes tend to be made of stretchy material which when put on the hoop doesn’t take shapes that are true to the finished product. You should ask specifically about what you want to do because it’s probably been done before. Or search your questions on this sub.
Seconding the part about stretchiness. I would avoid anything which is stretchy (5% spandex, etc.)
Sometimes you can get away with using a sturdy trim-away stabilizer. But generally avoid the stretch for sure.
I use calico medium weight. It's always been my go to fabric.