The rule is, as just posted, focus.

It's usually easier to focus when you use the carrier whose hub is at your home airport. However, it's not a rule, it's just a fact, and it's a generalization that doesn't apply to everyone. If it doesn't apply to your travel patterns - if you can focus on a "better" airline for your purposes without totally screwing up your travel - don't fly the airline whose hub you're at. It's your money, your choice.