In my experience here on the Maine coast, all the Firm's I've seen have been on NGVD 1929. Not all maps have references to BM's in the area, but those that do reference 1929. Hope this helps.
As for using Google Earth, I assume you're referring to the field on an elev. cert. that asks for a Lat and Long for the site. I've used Google Earth in this case, as they are only looking for an overall position of the site, and not for sub-centimeter precision control.