I think shifting depends on many things.

1. If you are going up to a hill or mountain, you should use proper gear, but make sure you're not in the redline.

2. If you are going downhill, stay in the lower gears (engine break) rather than using your brakes.

3. If you are in a stop-and-go traffic, smooth, laid-back driving is what you need. Upshift as soon as possible.

4. If you are passing, downshift to the next gear. This will give you better power & fuel economy ratio than squeezing your gas at top gear.

5. Driving aggressively will affect your fuel mileage. Try to be familiar with your engine's power delivery. Go to the next gear as soon as you think your engine can handle it.