Like others have said, this depends on your kids’ maturity. You found them with friends the last time, and that was against the rules, but what were they doing? Were they doing anything worrisome? Or were they just hanging out?
Your older son called you to get him when his friends were drinking. This suggests to me that he is not into alcohol or drugs, and these are the major things that warp teenagers’ judgment. This suggests to me that he is likely to make responsible decisions even if he doesn’t follow all your rules. He might just see some rules as unjustified. I.e., he thinks he is responsible enough to handle friends over without trashing the house.
Another concern is whether the children have the life skills necessary to handle problems that might arise. Like @Supacase and her cat. Or what if the electricity goes off? Can they cook? Or will they just eat cereal and order out for pizza the whole time? Will they take the money you leave them for supplies and emergencies and spend it responsibly, or will they blow it all on some Wii game?
My parents left me home for days starting when I was 17. We lived way the hell out in the country, so it was hard to get around without a car, and there was little trouble to get into. Even if we had access to trouble, we wouldn’t have indulged in it. We were too goody goody.
From the little information you’ve given us, I think the signs are positive about their being able to handle it. But you are their mother, and you know them much better, and if your instinct says they will get into trouble, I’d pay attention to that. However, what I think you should do is make an effort to separate your parental worrying from your objective assessment of your children’s capabilities. If, objectively, you think they can and will handle it just fine, then use logic to overcome your instinctive worrying.