Like Ani said,
There are different use cases. If you want to make a reusable app - that can be put up on pypi for example - then you shouldn't add a custom user model, but you can connect to the user model instead (this was previously recommended in django with the "UserProfile" solution).
However - if you are writing your own project - that you will be building on in the future, then the first thing I do is always to create a new user model. The reason for this is that it's hard to change that if you already have started writing your project and have a database in place and so on.