An excerpt from a letter dated April 14, 1969:

...With reference to the question of the pronunciation, Ashkenazi or Sefardi, surely you know that there is also Yemenite pronunciation and others. The adherents of each pronunciation of course claim that theirs is the right one. As a matter of fact, in recent times there has been a growing opinion that the Ashkenazi pronunciation is a more authentic one than the Sefardi. At any rate, if you want my opinion as to which pronunciation you should use personally, my answer is that you should not mix pronunciations, at any rate not during the same prayer, or better still not even during the same day.

In view of the force of human habit, and especially inasmuch as prayer requires concentration and heartfelt devotion, it would be well to get used to one pronunciation, and to abide by it, at least insofar as prayer is concerned...