Well, whether you might want to get the locks re-keyed or not probably depends where you would have lost them. If it was a far way away from home, so that anyone who picked them up, likely wouldn't be comming anywhere close to your vehicle, no biggie. If you were to live somewhere around Detroit, and there were several "chop shops" in close vacinity to your place of residence, you might
Anyhow, if it makes you feel any better, I lost one of the keys into my car (well I do have 2 pairs), but I know exactly how it happened. A long time back, after having locked the keys in the ol ignition, while going down to uni to study for finals, I had to call a lock smith to break me into the thing... Anyhow, they're expensive, and well finals week, after one's gone several days without sleep, been busting the books, etc, it can happen.
So, since that time, I put the spare key to my door/ignition in my wallet. All good and fine, and it did spare me a call or 2, but the key's head is also bulky with this big honken plastic piece about the top. The thing, well didn't want to stay in my wallet, but could easily slide outa the slot for it being so over-sized in thickness...
Well, one day, and without thinking, and it was well over a year, perhaps 1.5 years latter, I totally forgot what it was, and when it slid out that time thought I had an old key. I chucked the thing in the trash, and only the following day remembered what it was too
Yeah, I know, silly of me, but what happened
Oh, and yes I have had some old keys that have remained around in the past, for things I no longer owned/didn't even remember what they belonged to if anyone's wondering...