I think that, at least up to 9/11, the checks run by private companies were mainly concerned with criminal behaviour ... theft-type things, although murder might be something that put them off, too.
When any of my friends applied for gummint jobs which entailed security clearance, all their frinds, including me, would get a visit from the FBI asking really very superficial questions about loyalty, etc. These guys were so straight-arrow, they had trouble going around corners. Naturally, to a bunch of anti-authoritarian and (Blush!) somewhat sophomoric college kids, it was an invitation to lie outrageously to them. Which most of us did, at one time or another. I suspect they knew. From speaking to my now-defunct father-in-law, I know that his generation thought it funny, too. I don't know if the FBI thought it was funny. Them sumbitches never cracked a smile.