depends for me......how do I want the quilt to look after I am done.....? If I want a crinkly puffy look....sometimes I will not prewash as the flannel will shrink around my quilting lines and puff up the middle...but if I am using a lot of flannels from differant manufacturers.....I usually will prewash as I have found even quilt shop flannels will shrink up different amounts....I my experience moda flannel shrinks the least of the flannels and has a firmer weave....I once made a rag quilt with some moda and some other well known brands....well...even though I clipped the raw edges the same way...that moda just did not unravel and fray like the others....so I got little curls of fabric instead of the nice fluffy edges. I did use a chain store flannel for a backing on a scrap flannel quilt I did....knowing the chain store flannel was likely to shrink a lot I did prewash it....when I prewash anything....I do the same as I would washing anything...throw it in the washer on the regular wash and pop it in the dryer ....I also do that with each finished quilt....I want to know it is going to make it through a regular wash and dry cycle as I make quilts that are going to be used....rarely do I make an art quilt....would treat that one differently.....