A bigger variable, and one that nobody talks about much, is the moisture content of the wood before soaking, steaming, etc. Lee Valley has an online brochure on steam bending, and they say this about that:

1. The wood must be plasticized. Although wood can be plasticized chemically or even by microwaves when in a green state, the most convenient way to plasticize wood is with steam.

Wood cells are held together by a naturally occurring substance in the wood called lignin. Imagine the wood fibers to be a bundle of rods with the space between them filled with lignin. The strength of this lignin bond between the rods can be decreased by subjecting the wood to steam. With unpressurized steam at 212° Fahrenheit, steaming for one hour per inch of thickness (regardless of the width) will soften the bond enough for bending. Substantial oversteaming may cause the wood to wrinkle on the concave face as the bend progresses.

2. Only air-dried wood of an appropriate species should be used.

Kiln-dried wood must not be used; the lignin in the wood has been permanently set during the hot, dry kilning process. No amount of steaming or soaking will weaken the lignin bond sufficiently for successful bending. The same applies to air-dried wood that has been allowed to dry and stabilize below 10% moisture content; the lignin will only partially plasticize with steam, not enough for successful bending of anything beyond a shallow curve.

3. Wood must be kept under compression during the bending process.

Because of this third requirement, the Veritas line of clamping equipment for steam bending was developed. Wood fibers will stretch only a small amount before they fail, usually less than 1/2 of 1%. If you think of bending a stick over your knee, as the wood bends, it is the fibers on the outside of the bend that start to separate and break first. The drier the wood, the easier it will break. However, when well plasticized, wood will compress to an amazing degree. It is these two properties of wood that we avoid and exploit respectively in the steam-bending process.