I'd definitely open it up and take a look to make sure caps haven't leaked everywhere before powering it on. That stuff's conductive. You may do more harm than good turning the machine on if there's ooze all over the place. You can short out surrounding components. At least make sure it's clean inside. Make sure the PRAM battery hasn't leaked everywhere too.

As standard practice, I always open and inspect the inside of a vintage Mac before powering it on for the first time.