When handing off files to your printer, remember that the prepress staff have not seen the artwork before and will not know what it looks like. Desktop printer proofs, PDF or JPG proofs should be provided so that prepress staff can check for type reflow, font substitution or other issues which may arise.
My current employer has a standard operating procedure that if proofs are supplied single sided at 100% scale that they must be overlaid to the low resolution proofs which are output via our low resolution proofer. This is done by overlaying the client’s proof over the top of the low resolution proof on a light table, and any changes between the two are clear to see. When native files are supplied and there is a risk of text reflow, this proofing method is invaluable. However, overlay proofs can’t be done with files printed to a smaller scale; nor can they properly be done with files which have been printed on both sides, or a print has had alterations done to it since.
Alternately, if proofs are provided, make sure that the proofs supplied are exactly what is being submitted to the printer or bureau, and not an older version. If a proof with alterations marked is supplied, it must be clear whether the prepress staff have to make changes, or whether the changes have already been made. If colour is an issue, indicate whether the proofs supplied are colour-correct or not.