ORIGINAL: Zippi
You can seal both sides if you like but it's not nessesary. If your using CA hinges, the CA glue once applied during installation will harden the hinge so if any residue from the exhaust gets to the hinge it will do no damage. If it does then the hinges were not properly installed. Been doing this for over 15 years, never has a problem with sealing the bottom only. All it's for is to keep the air from passing through and causing flutter. It also makes for a more stable flying plane.
Flutter, yes, sealed gaps will help prevent this, But, consider this. I had flutter on a Cermark Pitts one time, when an aileron servo failed. The wings became a blurr like Humming Bird wings till I got things slowed down. It knocked one I strut mount loose and cracked the top wing, but she held together and I landed safely. The plane had CA hinges and all gaps were sealed top and bottom. All four ailerons held. Not sure they would have, had the gaps not been sealed and on both sides.
I have encounterd this three times over the last 38 years. The event above was the most recent (about 4 years ago). Each time the plane was landed safely without losing a hinge.