Originally Posted by deepthought
Well which is it varnish or dont varnish the underside?
As LakeSuperior suggests, put at least a couple of coats on the underside of the table to help stabilize/equalize the expansion and contraction of the wood.
Too many unknown variables in your original question: Type of wood, age of table and/or age of wood, what climate do you live in (hot and dry, cold and damp), varnish being used, application process, etc.
So, just put 2-3 coats on the underside to be on the safe side and be done with it. And no, it doesn't have to done the same day, just be careful where the bottom meets the side of the table (drips and holiday areas). Then move on to the next varnishing project