I'll be building a Timber Frame home , enclosed with SIPs. There will be 1 x 6 tongue and groove boards going on the frame where the ceilings are vaulted, prior to the roof SIPs going on. My question is...should the inner surface of the boards have a finish on them (e.g., polyurethane)? If they don't, will the boards look less than attractive after a while? Thanks.

Well if you don't finish them, they will eventually collect dust. If they are not smooth, they may be difficult to dust/clean...