Re: What spell rotation should i use with a lvl 50 destro lock?
Assuming you are fighting something that is not gonna die in 20seconds, what he said pretty much. But you really shouldn't bother with rotations until you have all your spells (70+ most likely) because you will find your 'rotation' changes when you get those spells (take incinerate for an example). Just get a feel for whatever is better at the moment.
Some examples: if you are doing brd with a bunch of melee in you group, curse of elements is not as valuable as if they were all casters; if mobs are dying quickly, curse of doom is a waste though it would do more damage than curse of agony (in your case that is, and after level 60 ofc); if you have a weak healer in you group you might wanna reduce life tapping in combat, leading to 'dps versus mana'...
Put simply, don't worry about rotations... know your spells and it comes naturally, you will be a better group member.
edit: There's an addon called DrDamage that might give you a hand at first... it's numbers are outdated, but are pretty close to accurate for the most part.