I think all the Arduino supplied libraries are written in C++ and any sketch using these libraries is using C++ constructs.
If you are more interested in interacting with digital and analog i/o, then the C language with the Arduino abstractions can get you pretty far. But if you are seeking to implement larger or more complex problems, or just want to learn to use the rich expressive power of the C++ language in order to become a proficient programmer, then time learning C++ would be an interesting and worthwhile journey.
I think a good approach is to concentrate on the core C functionality to get your early sketches going while reading and absorbing the underlying concepts of C++. But feel free to jump in the deep end if you so desire.
Anyway, have fun!