setting working directory in pycharm

[Larz60+]

The only thing this statement does is assure that the starting directory is always set as where python script is located.

do i need to keep this statement in all .py files

That's a qualified no, with qualification being that order of execution may cause path changes.
Safe answer is yes.
If you follow snippsat's advice, you shouldn't need it at all.
I always include this statement, it's only executed once, and I always know what my current working directory is.