There are two types of premium denim out there:
There's the "treated" denim which covers all those jeans that have been washed and faded and all those other crazy things that are done to jeans to break them down.
Then, there's raw denim; which is untreated and kept beautiful and untainted in its natural form.
There are a few reasons why you shouldn't (don't need to) wash your jeans.
1. *Most Important* the indigo will fade much quicker. Jeans bleed naturally, the more you wear your jeans the more the indigo will fade in the areas that get worked the hardest. If you wash your jeans that natural process is ruined and you won't be able to salvage them.
2. Raw denim is usually rigid and is coated with a resin that helps keep the indigo from bleeding. If washed that resin melts which may cause the jeans to bleed more easily.
3. Denim is a tightly woven material which loosens up after each wear. When you wash your jeans the structure of the jean is changed.
That being said, treated denim can be washed more frequently than raw denim. Seeing as that treated jeans have already gone through prewashes and wearing stages.
Raw denim on the other hand should be worn often and washed only after 3-4 months of wear if you can hold out go 6 months before you wash them. When you're washing your jeans you should only wash them in cold water with minimal detergent, and HANG DRY them NEVER put them in the dryer.
Of course you can also dry clean your jeans.
there is one exception when it comes to getting your denim wet. If you buy a pair of raw denim jeans you can always where them in the bathtub or place them in the bathtub in cold or luke warm water. This is process is known as "shrink to fit." This is your the only time it is ok for your jeans to be in contact with water right after purchasing them :)
Always make sure you're wearing your jeans a hell of a lot more then you're washing them. Treat your jeans like they are a part of your body, when worn and cared for effectively they become a story book. Each owner wears their jeans differently so each pair of jeans tells a different story. That's what makes denim so exciting.