I don’t think this is good advice. The point of a reset file is to reset everything. You don’t want to add an element later and have that element contain random default styles. Is understand how each line of a dependency works really important?
If you’re using one now, yeah it’s probably a good idea to understand what it’s actually doing, rule by rule.
I pointed this out to them all when I first joined, and they said, "But CSS resets are good ideas."
So yes - please take the time to understand what your CSS does, especially when working with tags that format text, as it can have subtle (and not so subtle) impacts on data created in a UI with unwise CSS reset choices.