Contrary to what some people say, you can get wrinkle free muslins... without ironing. 
For raw, bleached and dyed muslins (as long as they are colorfast) you can wash them. Then you tumble dry them on time (not the mosture content setting) until they are still just a bit damp. Then you hang it up on your background stand and let it finish drying. It will dry almost wrinkle free. Or course you can't do this with painted muslins, although you can wet down a painted muslin with a spray bottle or damp sponge.
To get out creases and folds, is lay a new muslin out in my backyard (as long as it's not raining) on the grass during the day and into the evening. I bring it in just before or after it gets dark so critters don't deside to make a bed out of it.
What this does is the sun shines on it, heating it up, which causes moisture to seep up from the ground. In the evening, when dew is starting to forum, the moisture will form on the muslin. All of this helps to ease out the creases and folds and even wrinkles. I then hang it up on my back ground stand. Sometimes I'll put weights on it to let it hang straighter. Only a couple of times, with painted muslins, have I had to repeat this a couple of times, mostly to get out the new paint smell as well as a couple of stuborn creases.
Once you get the creases and folds out, if you really want wrinkles, just wod them up and throw in in a corner or stuff them into a stuff sack. I perfer to have minimal wrinkles myself. So I will very losely roll them up or leave them on the background stand and just put my working muslin on top of it. Most of my muslins are in the 10x20' size also.
Mike