I would say that grabbing rails is a good thing, as long as your fingers don't wrap all the way around the rails.
If they do then this can slow you down and might leave you hung up in the lip! The problem is especially noticable when you wear gloves.
My advise is to place the palmes of you hands on the deck and hold onto the rails with the tips of your fingers.
This will reduce the drag whilst still allowing you to have full control. Dan