For me, I determine this on a course by course basis. More times than not I will go by the posted par on the tee sign unless it's obvious that the posted par is intended for a beginner level player. Good examples of this are my two home courses, Dorton Park and North Cabarrus Park. I play those as all 3's (but will be disappointed if I don't get 2's on several of the holes).
I also play Kilborne as all 3's but there are two holes that are posted as par 4. One of those holes, hole 12, eats me alive. As of today my stas for that hole are 0 birdies, 2 pars, 12 bogies, 2 double bogies and 1 triple bogey. Those are based on playing it as a par 3. If I played it as the posted par 4, the stats would be much easier to swallow; 2 birdies, 12 pars, 2 bogies, 1 double bogey and 0 triple bogies. However, I choose to play it as a par 3 because I know that it's within my reach that my game will improve over time and the stats will reflect the improvement.
Courses where I always play the posted par and probably always will are Renny, Nevin, Hornets Nest and Elon. My game has a long way to go before I can even think of achieving a 3 on many of the holes that are posted something other than 3 on those courses. These are championship caliber courses designed by knowledgeable course designers. As an intermediate level player I feel the posted pars are realistically attainable for an intermediate player.
I track all my scores and post them in the Scorebook here at DGCR. I'm the personality type that likes to track my progress so that I can see trends in my game over time. I understand how many people will play everything as a par 3 for the simple fact that it makes it easier to keep track of your score without having to write it down. But for myself, keeping score on each hole and recording it provides me the granularity to evaluate my weakest areas on a given course. For me, the posted par on a difficult course serves as a goal to be reached.