Hi @Gio Bacareza,
There really in my opinion is not set guideline.
The way I use testmy.net is to first test the closest server to my current location. Getting a baseline average of how the local or regional flow reacts.
Then I'll branch further outward, testing the ISP peers to verify how well their infrastructure is connected, quality wise.
With this, over time I can determine network loads, time of the load if any, and availability.
At the same time I'll test the furthest locations of the mirrors, this accomplishes how well the top tier ISP peering is connected (backbones), or main arteries connecting global networks.
It takes quite some time to get a rational image (as in records stored in your personal database here at testmy.net) as well as a mental image of network paths, usability as well as an availability baseline compared to others in your area, and worldwide. You can however shortcut to what is already in the public database, [https://testmy.net/country] which is compromised of all tests taken at testmy.net on a granular level of geography, country, ISP (and much more) for informational purposes to use against your test results.