re: How level do you think shop floors need to be?

IIRC, I think that the rule of thumb is that a floor should be within about 1/4" over a 10 foot span to avoid rocking.  That might be more for houses than shops though.  I would think that the larger the footprint of the machine the more noticeable it would be?  To check, find a longish board that has at least one good straight edge and lay it down to see where there are gaps and measure the height of the gaps.  You could find both peaks and valleys and the board itself might teeter on one of the high spots, depending upon how long your straight edge is.  It could be that grinding a few high spots might fix any problem areas.