I really want to explore this issue further as I have never had it clear. From Yehushuah's teshuvah from rav Shammai it seems it is OK to cut before it is used for a mitzvah. This is common esp when putting in new parshiyos as sometimes cutting excess klaf can ensure a better fit. What about after? For example you are checking someoen'es tefillin and the parshiyos stick down in the shel rosh a little into the titurah which is not so mehudar but there is plenty of room on the top and bottom, is there any allowance to cut to ensure a better fit? or they are very tight and you want to make a looser fit by cutting excess klaf at front andor end of parsha to avoid future damage? I should think this is highly problematic but I would love a second opinion.