I have been writing:
#!/usr/bin/python3
# -*- coding: utf-8 -*-
But I believe Python3 uses Unicode by default.
The default encoding for python3 code is utf-8. See python's unicode support.
If you want to support python2.x in the same file or if you want to use a coding other than utf-8, you need that comment, otherwise you can leave it off without any repercussions.