I do this exact same trick and have been using it for years. It led to a couple of brief and somewhat awkward phone calls with local business owners when I asked them rather pointedly about them sharing my information with third parties.

I also take this one step further and have inbox rules to automatically send all promotional email (from sites I'm interested in) to the trash folder. If I want a coupon for a website I frequent, I'll just search my trash for the latest offers from that company. Google conveniently purges messages from the trash folder every 30 days or so, and I don't have to worry about a massive backlog of promos.