Yeah...you tell a CBSA agent to get stuffed, it's "none of their business"....wait a minute, I'll go pop some popcorn.....
Always declare, always be honest. Failure to do so can result in having fines levied, your vehicle being torn apart, and your name being put on a database for automatic scrutiny each and every time you come back into the country from abroad.
But if you're honest and forthright, even if you are over the limit, or made an honest mistake......it goes much easier.