Do not ever tell a post office clerk what you are mailing. As long as it's legal, it's none of their business. If you have a printer, you can label a package yourself and you won't even have to
deal with human. You will even save a buck or two. USPS.com
Also, it is often a good idea to dig a little deeper on a subject.
443 Packaging and Marking
443.1 General
All sharp–pointed or sharp–edged instruments such as knives, tools, ice picks, razor blades, etc., must be securely packaged in a strong container as required in DMM 601.12.5.3 [in the original source this is listed as 602.12.5.3 which is incorrect]. Sufficient cushioning material must be used to protect the sharp points and edges from cutting through the outer packaging during normal Postal Service handling.
12.5 Knives and Sharp Instruments
12.5.3 Wrapping
Sharp-pointed or sharp-edged instruments such as knives, tools, ice picks, and razor blades, that are otherwise mailable, must be wrapped to protect their points and edges from cutting through the outer carton in which they are mailed.
I don't know about you, but that sure sounds to me like it's legal to mail knives.