I usually wash with color catchers first. I think the quality of batiks can vary, so if I was mixing better quality with not so good quality, I'd definitely wash.
I think there's enough pattern in the fabrics and design that you could use muslin and it would look fine. I like the 200 count premium muslin. It feels a bit nicer to me than the cheaper stuff. I got some Legacy on sale at Joann's last week, came out to $3.50 yd. Nice! There are some pastel batiks with very little design...think Hoffman 1895's like this one:
http://www.hoffmanfabrics.com/SuperS...ProductID=2499
or the new connecting threads one called Plumage, dove color. I don't think the little bit of "pattern" these have would distract from the block.
http://www.connectingthreads.com/ima...nails/4470.jpg
Be sure to come back and show us your quilt!