As soon as you are doing something that anyone relies on and to save you endless effort you should use South, IMO. Even if it's a one-person project (I have a few of those) South saves you both a lot of time and aggravation.

Aside from being a great habit to get into, South (or whatever other migration tool you want to use) will keep your database consistent and predictable to the framework. If you add or alter columns on your own, you run the risk of slight differences between what you've done and what the framework would've done left to its own devices. You may not have done anything wrong, but you don't want that inconsistency.

The biggest upside to it is that it fully documents all of your chances, and even allows rollbacks if necessary. Editing the database directly doesn't give you any kind of change log unless you do it manually, which is not really sustainable, even while working solo.