All denim is either sanforized (pre-shrunk) or unsanforized (in its raw cotton state). Soaking either type, if you choose to do it, has its benefits and can be an important step to undertake when starting a new pair of jeans.
Soaking Sanforized Denim
The main reason to soak sanforized denim before wearing is to remove some of the wax and/or starch in the finish of the denim. The soaking method is simple: fill a bathtub with cold water, turn the jeans inside out and submerge the jeans for an hour. After an air dry the denim will still be rigid, but will break in quicker with the removal of the starch.
Another rationale for soaking sanforized denim is to “set” the indigo dye to reduce crocking (indigo transfer). This is accomplished by filling a tub with cold water, one cup of white vinegar, one cup of sea salt and soaking the sunken jeans for one hour. Rinse in cold water and air dry.
Soaking Unsanforized Denim
The principal reason to soak unsanforized denim is to shrink-to-fit the fabric. Since the cotton is in its raw state jeans made from unsanforized denim can shrink 1-2 waist sizes and 2-3 inches in length.The method is the same as above except for unsanforized denim hot water is preferable (40°C/140°F).
In both cases, it is very important to keep the jeans out of the dryer and to air dry only.