The base58check encoding has a 32-bit checksum built in, so if you change one or two letters it is not only possible to detect the error, but to recover the correct key as well.
Armory even uses that for its paper wallets. You can make a 1 letter error per line, and it will work out what you should have typed.
For paper wallets, the highest risk is losing the piece of paper. Someone breaking into your house and finding the piece of paper is possible but much lower odds.