How to decide whether a mistake is sufficiently relevant to publish about it? How do I know my results are significant enough to be published? 

I am a physicist and I have found a very common mistake that appears in every paper in my area of interest. I thought it would be nice to let the people know it should be corrected. However, it is a very simple mistake, very basic math. I thought at first I could publish that as a short letter, but then I realized that I am unsure what actions the significance of the error(s) warrant.

How do I know whether an error is important enough to write a letter to the publication's journal?
You should talk to colleagues from that area of interest to check whether it actually is a mistake or maybe a common generalization that, although not perfectly correct, is still 'good' in the systems that were discussed in the papers. Also writing a mail to the authors of said papers is a better idea before writing to the journals without any cross-check.