In that case, as PIC, do whatever you consider necessary in the interests of the safety of air navigation.
If the instructor on board is rushing you or pushing you to do something you do not reckon is in the interests of safety, do your 'PIC thang'. If you're being coerced into to doing something your judgement says you shouldn't be doing, and you do it, you've failed the 'PIC thang'.
(If you want to know what's causally connected with the safety of air navigation, in the case of 'run-ups', let me know upon what engine and propeller your life depends.)