A piece of paper in a safe deposit box is, unfortunately, not risk-free. There have been some incidents where, for example, the owner of box #1343 loses his key and asks the bank to drill the box, so they drill box #1434 and give him the contents (hopefully your paper wallet does not say "MILLIONS OF DOLLARS IN UNTRACEABLE EASY-TO-STEAL CRYPTO" at the top!) Also, floods have resulted in box contents being destroyed (a bigger problem in some cities than others.)