It makes no difference in terms of grammar.
The reason for putting / at the end of a URL (not necessarily just a domain name) is technical, not grammatical. It has no connection with English.
Note that it only applies to a URL, eg:http://www.google.com/http://www.google.com/intl/en/options/
If you are just writing google.com (which is not a URL) then the / would be wrong.