Jeeze, we are planning to do mega shopping next week (just clothes and the like), it would have never even crossed my mind to declare my shopping.
I always use the Nothing to Declare channel and there are hardly ever any agents in sight.
Every now and again, I will see someone that has been stopped and their whole suitcase is open and being checked. And I once saw a couple that had about 10 bottles of alcohol so you know they got into trouble.