Washing stretch denim jeans is a bit more straight forward than the process for raw or selvage fabrics. Here are a few tips to follow to ensure the best results:
Wash them as needed
You know that rule about waiting 6 months to wash your jeans? You don't need to worry about that with stretch denim. That concept is geared toward raw denim (aka dry denim). The thought being that the more you wear them before washing, the more pronounced the fading of dye will be, giving it much more character. The same isn't true with stretch denim because, for the most part, much of the dye is already cured. For an indigo wash, you'll still have some dye transfer, but it will be dramatically less than a new pair of raw selvage jeans. All that to say, you can wash your stretch denim when they need it. We'd suggest every few wears.
Machines are OK
Due to the makeup of different materials in stretch denim (polyester, cotton, elastic), shrinkage isn't as big of a worry as it can be with raw jeans. You will still have some, especially with the application of heat (either from washing with hot or warm water, or popping them in the dryer). For this reason, we recommend cold water washes always-- preferably by hand. Hands are just more gentle. If you do want to add them to a washing machine though, that is OK. Just wash with like colors and cold water.
Drying Stretch Denim
Like any favorite article of clothing, hanging to dry is your best bet. Heat from a clothing dryer does crazy things, not to mention all the spinning and battering around. Will it ruin your jeans? Not immediately. But it will over time. Remember, elastic has a shelf life. After constant stretching, the elasticity will loose it's ability to 'snap back' to place. We chose our makeup of denim carefully to minimize this, but it's best to not push it by using a dryer. Rather, just hang them up. You can kiss them with 10 minutes of heat when they are almost dry to get some wrinkles out, but lets leave it at that.