Is there a standard or default projection on maps created by the US Geological Survey (USGS)?
The short answer is no, the projection used by the USGS varies with product (map series), application (paper vs web vs GIS data), and department/region/whatever. If you really want some detail, they publish a document on projections called Map Projections: A Working Manual you can find at http://pubs.er.usgs.gov/publication/pp1395
Tend to agree with @chris-w, if you look at the National Map, NED data) are not projected - though projections vary across the Survey, many times based on regional project needs (and if it's part of Mapping, Biology, Water, etc.). The USGS is spread across the landscape, typically with at least one office in each State, all with national, regional or local projects. As a result, each project manager may exercise discretion as to the projection defined - perhaps based on the need to better preserve area or other parameters important to the project. Water-related spatial data can be found through the WRD NSDI with accompanying projection information for each. A quick random scan of some of the layers indicate various zones of UTM seem to be popular.