Brant,
We usually leave this up to the Fab house becuase they know what they need
for their particular process and allow for it when setting up the job on a
CAM station. If you would require much more control over the final board and
have all of the knowledge required to CAM a job and have a good relationship
with your FAB House to know their allowances for their process then providing
CAD Data with this in mind might be what you need. Or the particular design
may require you to have that tight a control. A final note though if you
provide the CAD data with all the allowances in and say to the FAB house make
it like this, that should be what you get right or wrong, and if its the
wrong part you made the error. If you allow your fabricator to set up for his
process and provide enough information on the fab drawing to produce the
board you need, that allows the fabricator to provide the board to your spec
with his process and then the fabricator is resposible for meeting the fab
drawing and not you hoping you allowed properly. I hope this was not too long
winded and was of some help.