The only reason you should escape it is the fact that people have been known to redeclare the IP variable before hand to something that's not what you expect, IE
$ip = $_SERVER['REMOTE_ADDR'];
//some more stuff
$ip = 'some string';
//Insert
INET_* are actually used for storing/retrieving IP address so yes that's the better option.
@Palmer:
You have no understanding of mysql_real_escape_string() if you still believe that it inserts the slash into the database. ;)
So down to it all, either use INET functions or just insert as is or maybe even use mres it won't matter either way but if your going to do it at least do it right.