I remember when buying fabric in the 1960s that one always did two things:
- You bought extra because other bolts of the same fabric would have a different coloration; and
- You prewashed fabric for colorfastness.
When we opened Forever In Stitches in 2004, Thimbleberries fabric came with a recommendation not to prewash their fabric. On the other hand, some types of fabric consistantly run.
Fabric production and color control has progressed since the 1960s! So, whether you prewash your fabric or not is up to you. However, if some of your fabric is prewashed, it all should be prewashed to avoid irregular shrinkage between prewashed and unprewashed fabric when the quilt is washed.
Different fabric types [e.g.: homespuns and batiks] can have a wide variance in their shrinkage. We assume a 10% shrinkage in general.
We often see situations where quilters buy just enough backing to fit their prewashed tops, and then they prewash the back. The back is then too small.
Remember percent shrinkage is important and not an amount in inches. For example, 45" fabric shrinking 10% will loose 4.5". But if you are using 118" fabric, you could loose 11.8" and not just 4.5" because you are dealing with more fabric exposed to shrinkage.
Longarmers must have enough backing beyond the top to clamp onto in order to secure it to the frame of their machine. We require 10" extra [i.e.: 5" on all sides]. Other longarmers that I've talked to require that or more.
So inquire how much extra is needed from the longarmer you intend to use. We had a quilt brought in where the fabric and pattern were bought in Amish country. The sales lady said that a back that was 1" larger than the front would work [which is true on an Amish hand quilting frame].
If there is insufficient fabric, we sew on "sacrificial strips". These are 7" - 10" strips of fabric sewn on all sides of the back that are to small. And, yes, there is a fee for doing that.
Some longarmers just stretch the back. Some watch to try to keep damage from happening.
You can avoid all this by having the right size, squared back.
Remember: more work for the longarmer equates to more cost for you. Also, any time one uses a work-around, the end result may not be as good.
I think that it is safe to say that most longarmers love their craft. However, unlike Mary Ellen, they don't think that ironing, or sewing on sacrificial strips, or sewing seams shut, or trimming the three layers of the quilt so that they are square, etc. fits in that "love" category.