Bending down after receiving cataract surgery is a dangerous thing to do for the healing of the eyes. After the surgery, you'll be asked not to do any strenuous activities for a while, including bending down or heavy lifting something, doing sports, and doing other things. When washing your hair, your head should be tilting backward in the shower with low water pressure. The best thing would be to get your hair cleaned by someone in the sink so they can do it much more carefully.