Definitely talk to the customer. The last thing you want to do is wash it and she has allergies...doesn't like your detergent...the unwashed fabric bleeds....you don't need those liabilities. :)

My understanding is that you should never dry clean a quilt. :)

I can't imagine making a quilt for someone and NOT washing it. If there are any problems you want to know about it before the recipient gets it, so you can address any possible problems. Frayed seams, bleeding colors, stains,etc.

I'd wash it....

If the fabrics were not prewashed I would get some Retayne and follow the directions on the bottle to the letter. That product will lock in the dyes in the fabrics. Hopefully it would remove any soiled areas as well though it's not a wash product. Be sure and very hot water when washing and pop into the dryer asap. I usually dry to slight damp and then hang over a rack to finish drying.

To me washing it would be part of the "making a quilt" process.

I would definitely check with the customer before I washed it. Did you prewash the material before making the quilt? Even if you did I would also use retayne if I were going to wash it or have the customer wash it using retayne that you might furnish. After using the retayne, then you will need to wash it like you would any other one that just needed washing. HOWEVER, my suggestion to you and to the customer, whoever washes it after using Retayne, I would also put at least 2 color catcher sheets in the wash even if the fabrics were pre-washed before making the quilt ASK ME HOW I KNOW!. But I would definitley check with the customer first and get her wishes. You might explain to her that when quilting you do handle a quilt constantly, and even with clean hands and workplaces, body oil can soil any thing, and dust from the air, no matter how clean your home is can accumulate on any area. Don't dryclean, that might set the dirty corners in permanently.
I would try as someone earlier posted to wash with a damp cloth just one corner and and press well to see how that turns out, before making an effort to wash the entire quilt. You may just have to wash each corner. If washing one corner doesn't work, then definitely check with the customer before washingthe entire quilt!!
Can you show us a pic?

Pictures of the quilt and the corners coming Monday. Right now the plan is to use a damp cloth dipped in a solution of Synthrapol and cold water to see if that cleans the corners before washing the quilt; then washing the whole quilt in Synthrapol. I will let you know how it works out. Thank you for the many suggestions