My two cents:
I think in general the DM should play monsters 'to the hilt' and always go for the PC's jugular.
However, I think it's possible, if not likely, that dragons and other egotistical monsters won't be paranoid enough to play the way some of you are suggesting. Unless the dragon has advance knowledge that the PCs are a serious challenge, I think it's very plausible that it wouldn't bother to plan ahead or fight as if its life depended on it. To it, the PCs are just little short-lived two-legged twerps, and it's undoubtedly killed much more dangerous-looking opponents many times before.
Did Smaug recoil in terror and immediately fry a little hobbit who walked into his lair, or was he intrigued and completely nonthreatened? Did he fight the humans and dwarves in the most advantageous way, or did he (try to) destroy them in a way which would send them fleeing to tell the world of his terrible wrath?
Now, I think it's also likely that the dragon will have the sort of paranoid escape routes, traps, etc. available in case he/she turns out to be overmatched; but I don't think a dragon would use those tactics until it determined that it wasn't going to win the 'easy way'.
And finally, there's nothing stopping the DM from creating a truly paranoid dragon who destroys its enemies in the most pragmatic and efficient way. If you do that, however, I think it's worth letting the party know that, through rumor or legend.