I wouldn't advise going to the airport having overstayed your visa, even if it's only by a day or two. The Immigration officer might let you pay a "fine", but they could make things very difficult for you.
It's best to get an extension during your stay. If you're in a big city, just leave your passport with a large hotel or travel agent. They will be able to get an extension within a couple of days. I forgot how much it costs. Probably a couple of thousand pesos.
