Is Math GRE not worth it? Finished bachelor's and master's in mathematical finance. Applying for PhD in math, which may or may not be in a university that requires General GRE or Math GRE

Based on my other questions (like this) and what I read online, the General GRE and the Math GRE are taken only as required for application or taken with the intention of possibly applying somewhere where it is needed. Like a high grade helps little in admission but a low grade helps a lot in non-admission.

So showing a high grade in Math GRE to a school that doesn't require it in general would be all the more not very helpful in applications?

Let's say I apply to a set of universities in different countries where none of them require any GRE.

It seems to me that the only reason for someone to take for example the Math GRE other than being required is to strengthen your application based on your bachelor's or master's not being math.
The vast majority of math Ph.D. programs in the US will require both the general and math subject GREs, so unless you specifically avoid those schools specifically, you probably will have to take them.  How schools use the GRE varies a bit, but for someone whose record isn't as strong, it can play a valuable role in reassuring schools, since it's the only thing they have which is uniform across schools and recommenders.

For schools that don't require them, they will likely assume that you have taken the subject GRE, and will assume that not sending in your score means it is really bad.  If your score is really bad (less than 30th percentile), then it's probably better to not send it in, but if it's not abysmal, then probably you should send in the scores.