I always wash my jeans, then put them in the dryer for 10-15 minutes and let them air dry before I take them to be hemmed. This is the way I wash them too after they have been hemmed too. I NEVER dry my jeans all the way, this wears out the stretch part of the fabric. Brand new jeans will shrink a bit in length, so I think it's best to wash before taking them to the tailor. HTH!
I wash and dry them beforehand, exactly the same way I will after I hem them.
I usually wash mine in cold and tumble dry on low heat.