Thank you to each for your comments. Quilterpurpledog - thanks especially for your information, it’s exactly what I was looking for, just an explanation of what it is. :)

I would definitely suggest washing it before using, so that you won't be surprised by shrinking at a later point in your project. But I do that for quilting cotton also.

Be aware there are all different grades of muslin. Use premium quality muslin and I’m sure you won’t regret it.

I second this.

I have Moda extra wide in white and cream that is as nice as any quilting cotton. On the other hand, some of what we sold at JoAnn's when I worked there was thin and rough. Washing?? Of course. Last night I started to cut binding with two pieces of the same fabric - one washed, one not. There was an inch difference in the width. That's a lot in 40+ inches. Needless to say, I washed the second one. Timeless Treasures Fantasia, so not an inexpensive fabric.

I use muslin a lot and there are many that are extremely good! It is not all loose woven and see-thru. Kona Premium is one of my favorites