I really think it depends heavily on what type of congregation it is.
If it's a Sepharadi/Mizrahi congregation, it is surely more acceptable to pronounce words with at least the standard recognized pronunciation habits of Sepharadim (Tav pronounced as 't' rather than 's'; Kametz Gadol pronounced as 'ah' rather than 'aw'; Kametz Katon pronounced as 'o' and knowing when it's Katon; etc.) than to do otherwise.
When called to the Torah for an 'Aliyah, in many cases in Mizrahi congregations, there is an expectation that you will read your own section, and I'm fairly certain there is an expectation that you will pronounce the words "correctly".
In a Yekke congregation I have a feeling, though I don't know for certain, that there is a similar expectation, but with Yekke customs.
In Chassidishe shuls, I'm not sure what the expectation is, but I'd imagine there is a certain expectation that you will look Chassidish if you expect some Kavod, which would imply that those given Kavod will follow the local customs as well.
Note a common thread here. If one has an expectation to be called upon to lead anything, in some places one needs to look the part, and in those cases one would be expected to conform with with pronunciation.
On the other hand, any place that doesn't have such an expectation initially, might reach out to a visitor to give them the opportunity to lead Minhah, for example, in which case the expectation of conformity is already removed, and I would be quite surprised if they criticized someone's pronunciation for not fitting with theirs.
HOWEVER - on the last point, I would emphasize that this really only applies in a multi-cultural Jewish community such as one can find in places like the United States, Canada, and Israel, where many people of many backgrounds come to one place to pray. In certain parts of Europe, even today, as well as in parts of South America or other parts of the world, where all-comers are certainly welcome, the influence of the local community's traditions might weigh in the decision as to whether or not to "correct" someone else's pronunciation.