The function of the syncro is to match the speed of the intermediate shaft and the transmission shaft. The larger the difference in speeds between them and the harder you push into gear, the more it will wear.
Rev matching with the clutch pressed does help to smooth out the shift, but when you press in the clutch, the engine is no longer attached to the intermediate shaft and in between shifts, neither is the axle. So the intermediate shaft is spinning slower than both and it takes the syncos and clutch to match them all up when you re-engage the clutch.
Double clutching forces the intermediate shaft and engine to match speeds, which are ideally matched to the axle by the throttle blip, in that scenario, there is no syncro wear at all ideally.
So blipping is good, but double clutching is even less wear on the syncros. It is better to go through the gears as you drop down in order to minimize the speed difference if you are just blipping. If you're double clutching, drop all at once.
http://auto.howstuffworks.com/transmission4.htm
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