Rav Preida shows us good middot in Megillah 28a. He states:
לא ברכתי לפני כהן
I have not led a Zimun before a Kohein.
Now, we know that all of Bnei Yisrael are considered Kohanim (Shmot 19:6):
ואתם תהיו־לי ממלכת כהנים וגוי קדוש
And you shall be for me a kingdom of Kohanim and a holy nation.
Thus, Rav Preida is telling us to avoid leading the zimun.
For those of us without a convenient family minhag, what is the best excuse to not lead the zimun?