I’m at a loss as to why a student who is targeting schools where Subject Tests are recommended would not want to put together the strongest application possible - and that includes Subject Tests, certainly for Penn. Now if you took the Subject Tests and the scores sucked, don’t send them. But to not even try? SMH.
Subject Tests aside, if you’re not willing to take a risk, you’re not the applicant that a school like Penn wants, and you likely would not be happy there anyway.
Anyway, the only US college that requires all Subject Tests is Georgetown. For all other colleges, you have the choice to send what you like.