I suspect that overall, most people (>50%) who receive pre-employment polygraph interrogations ulitmately "pass." However, the failure rate varies widely depending on the agency involved. For example, in both the FBI and the Los Angeles Police Department, the failure rate is on the order of 50%. But in the Department of Defense (DoD) counterintelligence-scope polygraph screening program, everyone who does not make "substantive admissions" seems to pass (according to DoD's last two annual reports to Congress on the polygraph program).

I think that your suggestion, "If you go in and admit to something small (like smoking MJ) and then take the poly and stick to your story, almost always you pass" is ill-founded. While this may hold true for certain polygraph programs (like the DoD one), in other agencies, many truthful persons who make no "post-test" admissions nonetheless fail. See the narratives on the AntiPolygraph.org Personal Statements page for examples.