an obligation to be yotzei kol ha'deyos?

I saw something that struck me as very hard to understand in R' Moshe Shternbruch's teshuvos (vol 1 #18).  Without getting into the specific topic of the teshuvah, he makes the point that when it comes to fulfilling a mitzvas aseh, one is **obligated** to try to be yotzei kol ha'deyos and not simply rely on being yotzei according to rov poskim.  Rov, he writes, is sufficient when dealing with a lav, e.g. if you have a taaroves of kosher and treif meat, you can rely on rov and assume the meat is kosher.  However, when you are dealing with a mitzvas aseh, there is a special chiyuv of "u'shmartem is ha'matzos" / mitzvos which obligates a person to go the extra mile and ensure 100% that the mitzvah is done properly.

To back up this chiddush he refers to a Meshech Chochma at the end of P' Bo (here).  There is a Mechilta that says you have to check your tefillin once a year.  Mishkenos Yaakov infers from here that there is also a chiyuv d'oraysa to check the lungs of an animal to make sure it is not a treifa.  M.C. disagrees and distinguishes between mitzvos aseh and lo taaseh.  When it come to a lav, you can rely on rov because even if you end up eating maachalos assuros, the halacha excuses you -- acharei rabim l'hatos.  When it comes to a kiyum aseh, that doesn't work.  Even if you have an excuse for failing to do the mitzvah, bottom line is that you haven't done it.  You only get credit for what was done, not for excuses.

This is not quite the same thing as R' Shternbruch's chiddush.  The M.C. is talking about a rov in metziyus, the physical fact of whether the tefillin are damaged or whether the lung is punctured.  R' Shternbruch is talking about a rov of poskim.  The entire Shulchan Aruch is built on the premis of ruling based on rov when there are competing views!