Sure, it's says "optional" on your invite, but you have it on good authority that the event is a mega-bucks affair that the governor is planning to attend. In such cases, it's best to err on the side of caution and look your fancy-pants best by renting a tux or splurging on an appropriately formal gown for the occasion.
Women have it a little bit easier than men in terms of fashion flexibility. A floor-length gown is appropriate, as is a cocktail dress. Up the formality factor by choosing a dark color or metallic and something with embellishments, like embroidery, beading or intricate fabric work.
For summertime nuptials, men can feel free to wear a tux with a white dinner jacket. However, for the other seasons, a black jacket/tie/vest or cummerbund combo is the expectation.
If you're repeatedly invited to black tie events, you might consider purchasing your own tuxedo or picking up a few event dresses when you happen upon them (bonus points if they're on the sale rack). Chances are you'll save a lot of money in the long run by avoiding over-priced rentals or purchasing appropriate gowns at full price. Other guests might start taking a cue from your on-point attire, and you'll never fret about your wedding wardrobe again. At the very least, you'll look fabulous in pictures!