I agree that this is something more appropriate to do in your private shower at home, but let me just put this out there. Someone mentioned allergies earlier. Just so happens that if you take a really hot shower and there's a lot of steam present in the environment, that your nasal passages will relax and any spare mucous you've got floating around in there will start to drain downward. (That's also why it feels comfortable to take hot showers or eat spicy foods when you're sick to alleviate the congestion.)
I figure that if the snot is all but ready to pop out of your nose, hell, just quickly blow it. Blow it; get it over with. If you don't, you'll look like an idiot trying to just hold out til you get to the tissues. And you have to wash your hands and get the germs off anyway, so you might as well do it while under the hot water. Keep on wearing your shower shoes.