After hearing more discussion on this, I can look at it from both ways:
If I was in the lead (hell, or any position really), and it was the LAST lap, I would feel that I would want to block someone from taking my position. This is the final lap afterall, and it's the one that counts the most. Any other lap I would NOT block someone.
If I was trying to overtake someone and it was the LAST lap, if they blocked me from passing when I just took a better driving line or something and had more speed, I'd be pissed. If they blocked me while I was just slipstreaming them on the final straight, I'd understand and accept it.
And when I use the word block, I mean basically doing what it takes (within reason) to prevent someone from passing - i.e. swerving to prevent slipstreaming, or moving in front of someone trying to pass ONLY if they are passing due to slipstreaming. I would NOT try to crash them or touch their car in any way, and I would NOT block someone who just took a better exit out of a corner than me and was faster. If they get their nose past my rear bumper then that's that. I'm not going to smash into them and prevent them from taking me.
That's my 2 cents on it.