You will get more opinions, but these are mine:
I have found SOME muslins will shrink - a LOT!!!!!!!
I would put all the muslin squares in a pan of very hot water (as hot as it gets from the tap) with just a dab of detergent and just let them set until the water cools down. Swish them around once in a while. You can use a plastic or wooden spoon if you want to keep your hands out of the water. Then I would drain that water, and rinse them a couple of times with plain water. I would drain them and (maybe) roll them in a towel to get rid of some of the soggy wetness. Then I would just lay them on towels and let them dry. Some people use a salad spinner to get some of the moisture out of the fabric.
I would expect that there will be some shrinkage and wonkiness. I would then just trim the squares to the same size after they have been washed and dried.
I know- more bother than some would think it is worth. Uneven puckering/shrinkage/wonkiness drives me even battier. Pick your battles!
I soak all my dark colors (also in hot water) - like colors together, so in case there is a problem fabric, it won't be a disaster if it bleeds.
If there is a bleeder, I isolate it and rinse it some more. If it won't quit bleeding, I trash it. No need to give anyone else a headache!.
Others will swear using Retayne is the be-all and end-all for treating dark colored fabrics. I swear at the manufacturers that don't make color-fast products!
I think it is important to keep the agitation of fabrics to a minimum, because that is what I think makes them look "used".