“I know of a Rov who personally davened ashkenaz. When he took up a position in a shul where they daven sefard he switched: ‘Hashem understands all nuschaois, so I may as well daven the same as everyone else and not get confused'”
My teacher taught me that one should daven properly and try to choose a consistent pronunciation, taf or saf. But she also said that since many of us have a problem that we learned, for instance, krias shema ba’al peh in one pronunciation, then began the practice of davening in the other pronunciation, sometimes we get confused and end up using both during davening. (I frequently have this problem because my day school taught me to use taf but my husband uses saf, so I have been trying to retrain myself and it’s not completely internalized yet). She said that one should really try to be consistent, but if we accidentally stumble and say both pronunciations, not to worry because Hashem will understand us anyway.