I just keep playing. I think part of it is just my personality. Another part of it is feeling a sense of responsibility to keep the tourney moving. After all, someone has to knock off the short stacks. I've been in some where 100 qualify for the next tourney, sitting on a top 20 stack, and everyone but the two or three big stacks at the table are just in fold mode. None of us are gonna do anything stupid to put too many chips at risk, but we're still playing for small pots.
I would say that I would have to be in the top 50% of the field to feel really safe at any point in the tourney. Coming down to the wire with I would like to be in the top 70% to feel comfortable. I'm never really going to try to coast though. I might tighten up, but I tend to agree with "youregoodmate". With players stalling and folding anything but monster pairs, depending on stack size and blinds, and as some short stacks inevitably double up, you could find yourself blinded down and in trouble. Again, I'm not trying to do anything stupid, but I would rather play while my stack is big enough to have to be respected than to let it get down to the point where I have to shove and people feel like they can afford to try to call and hit a fortunate flop on me.
Again, let me reiterate, we don't want to be doing anything stupid. I've actually been in a satellite where top 100 qualify and was in around 20th, when a guy who was somewhere around 35th bluffed his whole stack off to me after I flopped top set. I remember wondering WTF, thinking that he could have easily coasted to the finish.