I've always at a minimum tacked my quilts. There is no other way of avoiding the batting shifting inside when laundered otherwise. As for how far apart you need to "tack" the instructions on the batting should tell you. Some say every 6 inches, others may be 10 inches, etc. In the beginning I used yarn and tied them in the appropriate spacing, now I longarm everything... :)
If your using fleece I'm not sure how far apart you should tack. Hopefully some others will chime in.