I'd say it's always better to ask forgiveness. They probably won't find out anyway, so why waste everyone's time (especially your own.) You'll spend more time writing useless emails to HR / lawyers than working on your side project. The simplest thing is to just do your thing, keep it to yourself. In the very unlikely event someone brings it up, you just play dumb. (Your Tetris clone isn't infringing on the company anyway.)