You can mix washed and unwashed in the same quilt top, but you need to be aware of a couple of things.
Once quilted moderately, the batting will control shrinkage so variations in shrinkage of fabric are really not an issue. (It *is* an issue if quilting lines are far apart -- say more than 6" -- or if a quilt is tied; then you would not want to mix washed and unwashed.)
To prevent possible bleeds from settling into other fabrics, it's a really good idea to wash the finished quilt with Synthrapol in a machine that uses a *lot* of water. Synthrapol suspends dye particles in the water so they are rinsed away. You need a washing machine that uses a lot of water so that any bleeds are diluted enough. Domestic front loaders don't use enough water for Synthrapol to be effective; however, domestic top loaders and laundromat front loaders are fine.