Yes, it's a good idea to wash cauliflower before roasting it. Not only will it remove dirt from the veggies but it will also remove any pesticides that could be remaining. You can find pre-washed, pre-cut cauliflower florets at a lot of grocers, so if you did this you can skip the washing. Always make sure to dry your cauliflower before seasoning, this helps it have a beautiful, slightly crispy texture when done roasting.