Yeah, I've hardly ever really regretted a kiss. Maybe one or two.
And not to take a decent joke and ruin it with a lot of facts and stuff, but... actually, Googling the phrase "I should have kissed her" brings back 10,500,000 results, to 3,690,000 for "shouldn't". Doesn't make as good a T-shirt, though.
Looking back the past 25~ years or so I can say that there are definitely more "should" choice then there are "shouldn't" choices. Seeing as some relationships have ended & started based upon the "shouldn't" category I would not call them "regrets" as much as life lessons learned.
All of us, after all, are really just the whole of our experiences so I don't regret anything. If we look back on anything as a regret then it should be chalked up as a life lesson as to what to do the next time the opportunity presents itself.