RJL, 
While I very much agree with all the above, but I have a different take.  
Time was I did a lot of boat work.  Much of it was done when the boat was "on the hard".  It may have been set down only level enough for the water to drain off the decks.  This would cause me difficulties when I was working below.  
Tools or parts would roll off "level" surfaces.  This can bring your thinking to a halt real fast and lost screws can be a nightmare.  I once put a can of something (like glue or paint) down on a table and it dripped over the edge.  I was barely able to keep that from being a disaster.  Sailboats is doesn't really matter, but power boats want mounted things to be level when they are stopped and that is harder to do when you have no level reference.
So, level or not is really your choice, and I personally would suggest that you cut blocks and an extra step to do this right the first time.
Matt