Ok, sorry for the wait. Everything i'm going to say is specifically referring to DOJ security clearances. I can't comment specifically on a state government background check. But, there are several reasons that withholding information is a terrible idea. First, let me outline the security clearance process. When you fill out your sf-86 you'll be asked for the names and contact information of roommates, spouses, employers, co-workers, friends, schoolmates, and landlords. Then, after interviewing those people, they will ask those people to recommend other people that they can talk to, they will also walk around the neighborhoods and buildings you lived in and talk to neighbors. So, while you can control who they talk to, to an extent, they will talk to people outside of your control. And, what if you had a roommate who disliked you, or a schoolmate who accidentally gave them the info of someone who didn't like you, it's not out of the realm of possibility that they talk to some people who don't care about your best interest first. So, lying about something you did at a party a few times is risky, especially with roommates and schoolmates who you don't have control over and may not feel inclined to fudge the truth for you. So, doing coke a couple times at a party is something that could come back to you if there were other people around who saw you. You will also be subject to periodic drug tests.
Then, if caught lying, at best you can be banned from ever working for the federal government, at worst you can be prosecuted. I imagine it will also bring up issues at C&F. Note: if it's a really small omission they may give you the chance to come clean, but something like experimenting with coke will get you banned from federal government work. But, if you admit to something, anything at all, you are free from prosecution even if it disqualifies. To clarify, prosecuting an applicant who lies is extremely rare, but it has happened. Now, if you do pass the background check, but then it comes out that you lied a couple years later, you can be fired, sued for past earnings, criminally prosecuted and/or have the possibility of being disbarred or at the very least not practicing in any good position again. Again, these are worst case scenarios, but it can happen. And, at the very least, you will be fired and have a very hard time finding another job.
Lastly, to whoever said people don't admit it, you are wrong. You get grilled at these things and everyone who gets a job admits to everything. Most will have done something they were worried about. I had a friend grilled about child pornography till he felt so bad that he admitted to looking at a sorta nude pic of miley cyrus from before she turned 18. He also admitted to drunk driving. He got hired.
TL;DR - Lying puts you in a position that can ruin your career and the background checks go beyond what you can control. People admit to things, everyone does, just be confident and sincere.