I am relatively new to the game, but I think a lot of this depends on the course. There are lots of courses out there where there are par 3 holes that realistically should be par 2. Typically, this occurs on short courses where room is not available for 18 realistic par 3 holes. We've also seen courses where the pars are clearly listed for casual players (par 4 on all holes), with no pro par listed. Other courses, no par is provided, though there is a clear difference between one hole to the next (hole 3 should be par 3, and hole four is 500+' with several turns and should be par 4).
If you're playing a course where the pars are realistic for the holes, I think you have to play the pro pars listed, be they 3 or 5. If you aren't, then I guess the 3 par rule applies?
The difference between ball golf and our sport is that unfortunately, disc golfers are often left to the course designers (or lack thereof) to determine par. This is often a very subjective determination, and in some cases, the determination never gets made.
I think it would be cool idea if a forum like this could begin to establish realistic pars for courses reviewed on the site. I know this could lead to endless argument and discussion on certain holes, but on many holes on many courses an established par could be established. When reviewing a course, we could enter a par for each hole - clearly most of them would be 3's, but when most of us agree that a certain hole should be a 4 or a 2 par, that would stand, as least with us. When you would pull up a course on this site, and look at a particular hole, the average par rating could be displayed - if a bunch of people rated the hole par 3, and one rated par 4, the average rating would come up "par 3.08," and we would all play this as par 3. Just an idea.
Until then, or some entity takes responsibilty for consistently rating all courses, I will play pro par when available, and make it up as I go along otherwise