If the curtains are lined, you do run the risk of the lining shrinking (if it does) at a different rate to the curtains which may make it difficult for them to hang nicely afterwards. . However, I wash my lined curtains in the bath, using gently warm water and soap and just swishing them around lightly. I then let the water drain well away for an hour so that the worst of the dripping goes down the plug hole and hang them in the open air to dry. You need to choose a good drying day if you have big curtains, and a strong washing line but this technique works for me.
I wouldn't risk using a washing machine as the treatment is too rough for some types of curtain material.