I'd do it for the NB heatsink if its been heating up a lot lately. The nForce chipsets are notorious for running hot, so if you don't have any spare TIM laying around, you could just strap a small fan to it that would help cool it down better. I need to do it with mine personally, its running way too hot for my liking. For the video card, it depends on if you feel like the TIM is getting old or the temps are too hot from what they were when you first got the card.
Btw, overclocking voids the warranty just the same as removing the stock heatsink. Some companies don't mind if you overclock, but the majority do and the warranty is void the instant you overclock. Just fyi.