At the end of the day it's just a matter of judgement - if there was a simple rule Adobe would have come up with a "content aware contrast" tool by now ;)
A very high level contrast is going to generate some instant "wow" factor that can be hard to resist when people are first learning how to edit images. Often the results come off as brash and overcooked. It takes time to develop a certain amount of subtly, and be confident that you don't need to crank the contrast up to 11.
I would start by asking myself is it even a good image? Will contrast enhance what's already there, or am I trying to compensate for failings in the lighting, subject matter or composition?
The next thing I would be would be to experiment. Certain subject matters and lighting patters do suit a large amount of contrast. Try out the various looks and see if you can make an informed decision on what is the best look. At the end of the day it's your image and your vision.
Here's an example based on a portfolio shoot I did a few years ago. The model picked the following shot, which I had set up with a fairly dramatic lighting scheme which usually benefits from high contrast post processing. Here's the processed image:
When I was processing another image from the shoot for my own purposes I immediately went straight for the maximum punch Dave Hill-esque band promo look:
But then something in me decided to see how far the other direction I could push the image. I could see quickly I was onto something, but it took me a little while longer to get it to look as good as I wanted. I think the extra effort was worth it, this image is now a benchmark I use when trying to decide whether to increase contrast. It looks a little flat compared to the other version, but if you come back and look at it in isolation I think it's definitely better, to me it looks more refined, and somehow more expensive...
I see various tips & techniques for "increasing contrast" and "limiting contrast." The techniques or straightforward; knowing when to use them isn't.
That's a very important point you raise there, and one reason I would advice against going to any one of the thousand websites that offer Photoshop tutorials for every effect under the sun. I think that by figuring out how to do things yourself you are more likely to develop a sense of when to apply certain techniques rather than how to apply them.