Yonatan M. comments:
Back when I was learning this sugya (I have since made a siyum ha-shas), I thought it was strange that Rav Ashi would answer that it was to guard the tefillin, if that wasn't his real reason. Even if he suspected that his answer would be taken as the halakha (i.e. he sensed that the conversation was heading aliba de-hilkhata), and he didn't want to be moreh that way, does halakha V.M.K. go as far as allowing one to deviate from the truth...?
J. Milevsky
The Kollel replies:
Sholom Rav.
The Gemara clearly intimates that one is permitted to deviate from the truth - with a statement that is justifiable, to boot - in order to preserve the sanctity of the Tefilin. Why is that a problem?
Be'Virchas Kol Tuv,
Eliezer Chrysler