Thanks for responding. I would argue the opposite, that the browser is the safest place to sanitize because it better understands the context where user generated strings will be inserted.

An example where the server may not understand the correct context occurs in this utf7 attack (http://html5sec.org/#charset). The server sanitizes the user input as utf8 where it passes. The client forces the browser to interpret the input as utf7 instead, and the string that was harmless as utf8 now becomes active js.