Just a heads up since you're probably going to get a lot of replies to this effect.... but there's no reason to ever soak BPs. They don't particularly enjoy it (if you're curious as to whether they want to soak or not, give them a large water dish. My bet is you'll never see them soaking) but before a shed it can actually be detrimental as it affects the oils that build up on/under the skin to help with the shed.
The best thing to help a snake shed is increasing overall humidity a bit or introducing a humid hide; basically a hide stuffed with wet sphagnum moss or a box with wet paper towels inside if you can't get the moss.
Just an additional bit of advice; this comes from having to be extra-paranoid about stuff like this for my frogs, but anything like a sink or tub that has ever had chemicals (soap, detergent, ect) in it can be potentially harmful to an animal even if it's been awhile or you've washed it out.