Some guy said that it doesn't matter when we compile it, any of them will be recognised as C++ file.
But thing is we should make a rule for it when we work on certain scale of project.
So I found nice answer to my Q here on Stackoverflow.

Link: https://stackoverflow.com/questions/1545080/c-code-file-extension-cc-vs-cpp

GNU GCC recognises all of the following as C++ files, and will use C++ compilation regardless of whether you invoke it through gcc or g++: .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx.
Note the .C - case matters in GCC, .c is a C file whereas .C is a C++ file (if you let the compiler decide what it is compiling that is).
GCC also supports other suffixes to indicate special handling, for example a .ii file will be compiled as C++, but not pre-processed (intended for separately pre-processed code). All the recognised suffixes are detailed at gcc.gnu.org