Typically, polygraph screening is not required for positions that require only a secret level security clearance.

I don't think that would affect whether you are polygraphed. An agency that uses the tests for top secret clearances will usually disclose that you will be polygraphed when you apply and are asked about prior drug use. I would not recommend lying about past mj use, however long ago or minor. Lying on background investigation forms will cause you to be denied a clearance, if its discovered. I would say most people (not ALL, however) have experimented w/ mj during youth--not really a big deal unless you did it excessively or sold it. It sounds like you have nothing to worry about w/ a secret clearance--it only covers the past 7 years, I believe. I'd tell the truth and not worry about it. I WOULD worry about being COMPLETELY  honest about everything in a poly--that gets you into trouble.

I've had a Secret level DoD clearance for 20 years and have never had to take a polygraph examine.  I've done plenty of interviews in my PRs (Periodic Reinvestigations).  I did find out one thing, if someone doesn't like you and they get interviewed, it can make the PR a long process.  I'll explain in a post one of these days when I can sit down and type it all out.