I had one of those tiny titanium clips inserted 3 years ago... during my stereotactic biopsy. As it was being shown to me, I was told that it serves as a marker for future diagnostic referencing. So, I would say that it's necessary... in that respect. As for allergic reactions to metals, that's always a possibility; but, it's my understanding that an allergic reaction to this particular metal is rare. I'm also hypersensitive and prone to allergic reactions... and, so far, I haven't had any adverse reactions as a result of having the clip. But, your reluctance and concern is understandable; and, if you're truly against having the clip inserted, inform the radiologist and technicians of that decision.
It's your body and you can't be forced to have something implanted if you don't want it... for any reason. Good luck with the biopsy, "i"