yes, I agree with the others, but I recommend that you wash it in warm/hot water and then dry it thoroughly to complete the shrinkage. Muslin shrinks a LOT and often has residual shrinkage over the next few washings. It's best to get it all shrunk up first.
Even the "preshrunk" muslins will shrink.
Muslin comes in a wide variety of qualities.
So my opinion is that muslin works just fine after you have completely shrunk it. :-)