Dented frame = you're gonna have a bad time?

I wanted start riding again after five or so years, so I bought a beat up bike for $50. The thing is made of some decent parts (S&M, Primo, etc), but it's old, heavier than newer bikes and pretty beat up. I had a new chain installed today and the guy at the store told me to be careful because me frame is dented. It has a few small (1-2cm), light dents in the top tube and a slighly deeper ~2cm dent in the bottom tube. I was told that dented frames become weak and can break easily. Is this really true? I've been riding around and practicing stuff like fire hydrants and 180s, so I'm not doing any huge jumps like 10-stair sets or anything like that. Once in a while I might do some smaller jumps, but nothing extreme. Is it likely that I'll eat asphalt if I'm not careful or should I not worry about it too much?