I am told the CS bolt and washer are a consumable item - meaning they should be replaced every time you replace the front sprocket.
For folks who swap out front sprockes regularly, changing tooth counts depending upon the focus of the ride, I suppose they should change the nut and washer regularly but how often is a question best asked of the professionals (folks who talk about "bolt stretch" and metal fatigue...).
BTW, the bolt and washer share a part number and they come pre-treated with loctite. Of course if you swap tooth counts you should clean and reapply the correct loctite.
zerodog, I feel stoopid asking since you are an engineer/machinist, but you did use a torque wrench on that bugger eh? No over-torquing...