joseph: You are partially correct. The Shulchan Aruch (39:10) states that tefillin with a chezkas kashrus never need to be checked and those that are only used “occasionally” twice in seven years.
http://www.hebrewbooks.org/pdfpager.aspx?req=49623&st=&pgnum=134
I don’t have the exact quote in the sefer Minchas Shlomo but R’ Shlomo Zalman T”ZL states that since tefillin are made so much better “today” the chezkas kashrus is even stronger.