There are two kinds of people: Those who use a napkin to blot off pizza oil, because hey free calories, and those who laugh at the first group. How much difference could it possibly make?
Photo by Jenn Durfey.
Mental Floss rounded up a few estimates, finding rough agreement between Food Detectives (35 calories per slice) and Popular Science (20 to 50 calories).
I have access to both a high-precision scale and a healthy appetite for pizza, so I can provide a few more data points. (My method: Weigh the napkin, blot the pizza, weigh again, figure the increased weight at 9 calories per gram).
From an 18" New York style cheese pizza, delivered and thus partially cooled but still plenty oily, with a cheese surface area per slice of 130 square centimetres (I own a ruler too), I was able to take off 20-40 calories with the first napkin, and a pretty consistent 20 with a second. The difference in taste between blotted and un-blotted pizza was detectable, but only barely.
So the difference is small, but adds up over a few slices of pizza — consider that with three slices you can blot off something like 120 calories, roughly equal to what's in a scoop of light ice cream. That said: pizzas vary, tastes vary, and dedicated blotters run the risk of getting a napkin stuck permanently to too-hot cheese. So blot if you prefer, but blot wisely.