You can also use Flash Exposure Lock on the G-series, including the G6. It's much like locking the focus and recomposing the shot, with the addition of pressing the * button, after half-pressing the shutter.
You can also manually set the flash intensity in most modes.
I don't know about the A-series. Hit canon's site--they have all the manuals in .pdf. In some ways, browsing the manual of a camera you're considering is better than reading a review 

