This is a very fair question which I am debating myself now. So here is the answer:
1 - If you will NOT have too many users (under 5K) and IF you will not have too many additional properties, I would say that it is ok to use the custom fields that DNN allows you to add;
2 - However if you will have too many users (beyond 5K) and if you will have many custom properties, I recommend using a separate table of your own and link it to the users table via the UserId. This is because the way DNN stores the additional fields is not optimized for performance;
That is my 2 cents.