It's the only muslin I buy, and i buy a bold whenever I run out - from Jo-Ann's on sale. I wash as I use: I cut a piece several inches bigger than I need, serge the edges and wash and dry - using hot water - at least 3 times. I want it to get ALL it's shrinking done before i use it! It still needs pressing, but it doesn't wad up into a mess like regular muslin. Tip: serge the ends together, and it won't twist up in the dryer.
I'm a pre-washer. Many others on the Board are not, so you'll get both kinds of advice. It's pretty much up to you. I've found that muslin shrinks much more than other cotton fabrics. That's why I prewash it 3 times.