I'd prefer to leave this as-is for now -- we're very inconsistent about flagging nullability at all in most docs, so having it marked explicitly (even if matching the default) seems useful since it indicates someone definitely thought about whether it's nullable or not.  If we ever do a big sweep and clean up all our JSDocs, it seems safe to start leaving it unspecified when matching the default... but maybe not just yet.

One of these days I still want to write an extension that runs Closure on your code to give type safety warnings -- sort of a JSLint/Hint on steroids.  I think that would make it _much_ easier for us to actually use Closure annotations properly...  As it stands, I bet it's hard to find a file in our codebase that _doesn't_ contain at least one incorrect type annotation :-(