If there'll be photo close ups, I'd recommend a pro. I was involved in something recently where I had the opportunity to use a pro & declined. Boy did I regret that decision: My usual home-done makeup was awful on camera, and my pores looked like something the Mars Rover could land in. But my colleague who opted for the artist and looked slightly clownish in person looked gorgeous and very natural on camera.
I like Angie's and Diana's suggestions to get yours done with the bride. If she's worried, it might be calming to have you there too. In my limited experience w/ this sort of thing, what looks 'natural' IRL looks washed out & blah in photos. Since the photos will be the lasting memories, might it be worth the 'surprise' of seeing yourselves looking more 'done' than you're used to seeing? Of course if it's hot w/ lots of dancing, there is the downside of makeup melt.