My first reaction for metallics would be no, but lets wait and see what the printer/prepress contingent has to say. I was always told that you want to avoid polluting the metallic.
I agree with Peter and getting advice from your output provider is almost always the best route.
Metallics are usually more opaque than other solids, and there can be a couple of trapping issues you might need to be aware of, so here are a couple of resources that you might want to check out:
in this excerpt here: http://www.adobe.com/designcenter/tutorials/trapping/ (from the IDCS2 Real World Guide - still good info)
and this InDesign Magazine excerpt here: http://www.printingforless.com/InDesign-Pantone-CMYK.html
Hopefully of some use
You would in most cases NOT want to overprint metallics, since they will mix with the underlying colors and cause either the metallic or the underlying colors to react unusually. If you do want to overprint them, you would be best suited to do a "dry trap", in which the colors below are allowed to dry first, then run the sheets back through the press with just the metallic(s) printing. Ideally, you would trap the metallics slightly less than the other colors, to prevent as much mixing as possible in the trap area.
@ the OP
ask the printer in question.
if the artwork was coming to my business (a sheetfed offset printer) i'd say no, don't apply any overprint, only be aware that metallic inks are OPAQUE (hides any colour underneath it) and it is generally run on the last unit, so items like drop shadows or items using the indesign color [Black] (which defaults to overprint) should not be put over the top of silver.
but again, ask the printer - they will decide how to tackle the issue.
Thanks for all the replies, looks like the best bet is to find out how he wants the spot supplied depending on the print method..