You want to finish the quilt before washing, as the quilting will hold everything together. Soaking a top is extremely risky. For one thing, you do not want multiple fabrics soaking in water; if one of them bleeds, there is plenty of time for the other fabrics to absorb the loose dye. Also, fabrics shrink at different rates; you can end up with a massively distorted top which you can never get to lie flat again without tucks and wrinkles.
The first wash of the quilt is important if there is a risk of dye bleeds. The most foolproof method is to use Synthrapol with a ***lot*** of hot water. Synthrapol suspends loose dye particles in the water so they can be rinsed away instead of settling into other fabrics. You still need a lot of water to dilute any dye bleed as much as possible. Most domestic front-loading washers do not use enough water. I take mine to the local laundromat and use their largest front-loader (can handle up to a king size). Never let a damp quilt sit before drying; the fabric-to-fabric contact while damp will promote movement of dye. Remove immediately from washer and either toss in dryer or lay flat (with fans to speed drying if done indoors).
Synthrapol is often sold in quilt shops, especially those that cater to dyers. It is also widely available online:
http://www.amazon.com/Craft-Synthrap...dp/B000YZ3UHQ/