I agree very much with what Little Guy says.
It would be highly unlikely this would change the rankings much at all apart from separating teams on equal wins.
I checked quickly past 6 Nations tables. The only year it would have made a difference to the winner would have been 2007, where Ireland would have won on 19 points to France's 18 points, rather than France winning by a points difference of +69 to Ireland's +65.
It is very rare and improbable that a side would get the 5 bonus points it needs, and for the other not to get a single one for them to eclipse them.
England came close in 2002 though, they scored a bonus point try in 4 wins, and a bonus point loss to France. Meaning they could have won, but France also got one try bonus point so France would have won by head to head.
Also there would be an easy solution for this. Just making a rule so winning a Grand Slam equals extra 5 points would guarantee the Grand Slam winners the Championship.