Oil Undercoating (rustproofing) and DOT inspections?

Here in the salt belt, it's pretty common for people to do an annual "oil undercoating" on their cars and trucks, and it really really significantly slows down rust. Although, a lot of people I know have talked about driving down south in their personal cars, going to a mechanic down south, and having the mechanic have an absolute melt down "OMG! You got a major oil leak! We need to clean this up! I can't even tell where it's coming from!" So I am curious: Does anyone here do an annual oil undercoating on their rigs, and have you ever had any kind of issue with DOT cops because of it?