This is something that I have thought about a great deal with my own game. I am a Decision Scientist for a pharmaceutical company, and I believe that par should have little or no bearing on decision-making DEPENDING ON YOUR VALUES. If a person values low scores, that person should make the same decision for the same conditions regardless of how many he lies or what the par of the hole is (e.g., 235 out, ball above the feet, water left should result in the same decision process whether you are lying 1 on a par 5 or 3 on a par 4) as the risks/rewards are the same.
People often make the mistake of factoring sunk costs into their decisions. How many strokes it took you to get here is irrelevant to your decision--you can't get them back! The better way to think about it is to start counting from your current situation rather than from the tee. For the above example: "I could hit the green and two-putt to get down in three, but I've been hooking my 2-iron all day and if I go left, I'll wind up getting down in 4 or 5 at best."
This way of thinking works until you throw competition into the mix of values. Values change slightly as I not only want to shoot a low score, but I want to shoot lower than my best competitor. The riskier or safer shot becomes more attractive depending on where you stand on the leaderboard. Even here, unless you place a high value on moving up one spot or avoiding moving down one spot (e.g., the difference between 1st and 2nd at the end of the tourney) your best bet is to make decisions the same way you normally would given playing conditions and your shot alternatives.
Another example of how values can change is when you stop "protecting a score." I played Pebble Beach a few years ago and, honestly, my values centered around what I would be able to tell my buddy about my round afterward. I'd love to say "I shot 75", but when it became evident that I wouldn't be relating my 18 hole total (somewhere around #7), I started pulling out my driver more and shooting at flags rather than "playing smart". I wanted to be able to say, "Well, at least I birdied 13" (which I did). I sacrificed several strokes for my "trophy memory", and I'm glad I did.