I normally leave it until "decel" point and let FMGC activate it automatically, unless I have reasons to decelerate earlier (tailwind, interceping G/S from above platform ALT, etc.)
The only time I activate it early is when I need to fly selected speed below managed target (eg. ATC tells you to fly 220kt) to avoid unwanted acceleration when going back to managed speed.
Seing guys going selected speed at 50 miles out for no apparent reason is one of my pet peeves
