Here on Cape Cod (where I think white cedar shingle siding was invented) the vast majority of the houses have white cedar shingles on the sidewalls, we only use reds on the roof. It is almost un-heard of for anyone to treat them with anything. I have seen houses with 40-50 year old shingles still doing fine. I have seen some 20 years old or younger that need replacing though - only in the case where: 1. the gutters were letting the side of the house get wet for years, or 2. too many trees too close so that constant shade and/or no air circulation lead to some rot - even then, only on the north side. FOrget about sealing your shingles. It sounds like you have more important issues to worry about, like roof venting and insulation.