Do you choose to pronounce words in the original accent or as they have come to be pronounced over time?

Words like schism, Van Gogh, and forte have been so long and so commonly mispronounced that the correct or original pronunciation is now just an also ran. Is it better to go with the flow and secretly know the proper pronunciation, or to risk being seen as an affected pettifogger and pronounce them the old-fashioned way? Worse, risk well-meaning correction and then the dilemma of whether or not to explain that you really ARE using a proper pronunciation, thereby implying that the listener is a cretin?
Also, I’m wondering what is the basis of the tension here? After all, if the dictionary has allowed for the way the words have become pronounced in common speech, is it more polite to use the generally accepted pronunciation? On what basis do we decide how to pronounce these kinds of words? Manners? Linguistic purity or precision? Pettifoggery?