I would never "forget" to declare anything. I am always prepared to pay the border taxes on stuff bought in the US, and if they don't ask or wave me through, I consider that a "bonus". Oftentimes, as I'd said previously, even WITH paying the extra tax, the item is STILL cheaper in the US.
Failing to declare items, I am told, is grounds for seizure of the vehicle. Not willing to take that risk to save a few bucks!
I don't know if officially they allow averaging of values between people in a group, but in my experience that is how the line of questioning has always been by the customs agent, at least for my family when travelling together. They ask for the total value of all goods purchased, and we add it all up. So if one family member buys an expensive item and the others stayed well under their individual exemption limits, the values can average out.
True story: I went on another one of those bus tours with my family last month. As we approached the border crossing, the tour guide got on the PA system and coached the passengers in how to properly lie about how much they spent! The guard seemed to be used to that -- he entered the bus and asked for a show of hands, and when he got none: "Did anyone spend more than $400? *smirk* No, of course you didn't."