Everyone has their own preferences. The only thing that's really important is that it works for you. Pedaling too hard? - downshift. Pedaling too easy? - upshift. I like to think of the front gears as coarse adjustment and the back gears as fine tuning. I'll often just leave it on my fav gear in the back and ride it like a 2 speed only shifting in the front. Then fine tuning the back as needed to keep a comfortable load on the pedals. But, it's ALL about what's comfortable to you.
One thing you want to avoid when shifting is called cross chaining. That's when you have the chain on the smallest gear in the back and the smallest in the front at the same time, and vice versa. That makes the chain diagonal. It isn't very efficient and can cause premature wear to the chain and sprockets.