When I started in the architectural / engineering field as a "draftsman" we were always told not to scale drawings. Way back then, board drafting, you had to think what the dimension truly was before you put a number down for a dimension (IE it might measure 12'-2" but should the dimension actually be 12'-2 1/2" or 3" or ...) it goes back to the precision of the scale you are using which someone had previously stated.

Now with CAD we run a dimension string and suddenly we are through with dimensioning the project. But actually we need to look at each dimension and make sure that it is correct. Many times I have see that someone has snapped to something near what they were intending to dimension to, but it is not correct. Make sure your dimensions are correct.

Also, if the CAD operator dimensions the project properly, then there would never be a need to scale something on a hard copy. I see drawings from many different firms, and the art of dimensioning is either not being taught or not being learned.