Example: let's say that I have a rectangular part that I would like machined. I'd like to cut it to change the length, but the width and thickness are fine as is. The width and thickness are not nice round numbers. Is there any convention on how to communicate this through a technical drawing? Something like "AS IS"?
There are standard symbols to show which surfaces should be machined, and which should not.
See Fig 11.3 in http://www.engr.mun.ca/~dfriis/cadkey/program/textappi.html (or preferably, read a proper standards document!)
The way to indicate dimensions that do not have to be machined is with a so-called reference dimension. The dimension used to be followed by 'REF', but nowadays the dimension is simply placed in between parentheses, e.g. (1 mm).