Going through my memories of how thoroughly they looked at stuff at MEPs (former navy), I don't remember them looking all that hard. They're generally looking for reasons to let you in, not to kick you out.
However, depending on the job you're looking at, there may or may not be waivers available (if you wanted to go entirely on the up and up). I'd probably go with @Doll's idea of saying it was from some kind of abuse you received if it came to that.
Now, after you get in, and you need to get a security clearance, be completely honest. They'll look, and they have a decent chance of finding. You probably won't not get a clearance based on what you've described (it'll just take longer), but you definitely won't if you say you didn't and they find out.
I already told my recruiter I had those scars but they are old. Haven't heard back from him, but I did receive a call today from a Navy official wanting to ask some questions. I was at work so I'm calling back tomorrow.I wouldn't lie if directly challenged, but I wouldn't offer information about it unless asked.
The job I want requires TS so I don't wanna risk getting in with a lie only to be discharged after the investigation. Besides, how honorable is to serve your country under false pretenses? Not at all.