My first choice would be to finish the quilt and then wash.
If there is bias on the edges, I would be even more tempted to finish the quilt before washing and air it out well if there is an odor that bothered me.
If not, I really like the suggestion of using the sheet to manuever the top/or quilt in the bath tub. I like that you are not pulling at the top, the sheet bears all of the weight, and distortion would be minimal. Thank you for sharing this :D:D:D