As a newbie, should I just focus on one traffic source?

The more I read, the more it sounds like getting a good idea of good and bad placements is pretty important. To build up this knowledge, would it make more sense to focus on just one traffic source? I have a somewhat limited budget fwiw.

Also, do you create blacklists for widgets that are campaign specific, or do you create blacklists for entire traffic sources that you use across different campaigns?