I despise pre-flash.
With my Maxxum 7d the interval between the preflash and the exposure seems precisely calibrated to avoid red eye - because when the subjects eyes are closed, there is no red eye!!!
I wish that the flash units would have sensors in them. Not ideal, but better than pre-flash or ADI.
Often, for parties or other gatherings, I set the flash manually for a certain distance, do a check shot and then shoot at that distance and aperture only. Consistency at least. (f/8, ISO 400 and be there...)
Perhaps with the a900 the above will improve (I'm still on the 5600HD) and in no mood to spend on another flash. I'd rather buy more studio lights.