i look at it like trail ed. like skiing rules - uphill skier is responsible for overtaking or coming toward and avoiding the downhill skier.
if you're coming to a blind narrow turn and you are cooking, you need to scrub some speed to be able to control yourself and come to a stop - safely - in case there is someone/something around that corner. i'm all for having fun turning up the speed on a downhill, but if you don't know what you are heading into, you need to make sure you are under control, and not just 'holding on'. so if there is someone who's fallen or if there is someone coming up the hill you are descending, you need to make sure you can stop and control yourself enough avoid throwing them off their line, or even worse, making contact.
if its a place like lewis morris, where you get a great view of what is ahead on a lot of those downhills, you can and need to look ahead if you are turning on the jets - if its all clear, go for it. if you see another trail user in your line, adjust your speed until you are clear of each other.
if you have ever had someone come by you on a downhill while you are standing still and they come real close to you, 7 mph looks really fast... never mind 30 mph.
speed shouldn't be checked only in downhills... in group rides too. tailing someone really close is a bad idea sometimes - especially if you aren't experienced at riding on someone's tire (like in road riding). if you can't stop yourself in time to avoid hitting the person in front of you if they bail, you are too close. when the speed picks up, back off a bit to allow for reaction time and your tires to slow you down if you aren't used to riding behind someone like in a road ride. and if you are going to pass someone, audibly communicate it, and then commit to it and get by them. dont ride just ride their butt, and then take your time passing - turn on the jets and get it done.