There's a middle ground, Fairylea - and being welcoming to your teen's friends means you have a better idea where they are, and what they are doing, whereas if you aren't welcoming at all, they will just go and hang out with their friends somewhere else, and you'll have no idea what they're doing.
And I'd rather know that my teen and his friends trust me enough to come here when they are pissed, rather than wandering the streets getting into who knows what trouble.
But there are boundaries - like I say, there's a middle ground. All ds3's friends are nice lads - polite and friendly, and they don't take advantage. If they were rude or sullen, I would probably feel a lot less like welcoming them.
It gives me some reassurance too - currently he is off in Malia on his end-of-schooldays holiday, with his four partners-in-crime, and six other lads from their school. Of course I am fretting about too much drinking, sex with strangers, injuries from quad bikes or balcony-jumping etc etc, but because I know his friends, I know that they are going to look after each other, and I trust that they aren't the sorts to go out looking for the worst sorts of trouble - and that is giving me a measure of reassurance.
Mind you - I will be a lot happier when we pick them up at the airport on Friday night, and I can eyeball him for obvious injuries, and start detoxing his liver!