Table top finish

Question for finishing table tops, Most of our tops are about 2” thick and have a natural edge to them, We are in northern michigan, I use the rub on minwax satin finish, should i be coating all sides including the bottom to protect against humidity wrapping. or should the bottom be done with a sanding sealer, what the best attack at this,
I have a walnut coffee table that has been in the home for about 2 years bottom is unfinished, we have had a lot of rain over the last month, and now it has develope a end check and a rasied crack.
Help
most of our tops are cedar or walnut. some pine

-- Doug Cedar Log Furniture, www.cedar-stuff.com