Katoman has your answer. I built my shop 11 years ago (dug out a half basement under my barn and installed a wooden floor). I secured my bench legs to the floor joists so it would never move and although it's not perfectly level, it is perfectly flat because I used a solid core door.
I never touched the top until last week when I gave a face lift by installing a bamboo flooring on top. It's still flat and I love flat for my workbench.
Its' never hot or cold in New Hampshire... its' always seasonal.