To flesh this out a little... all stats taken from
http://stats.rleague.com/rl/rl_index.html
6 teams have taken a week off and gone on to win the title:
2000 - Broncos (1st place)
2001 - Knights (3rd place)
2003 - Panthers (1st place)
2007 - Storm (1st place)
2008 - Sea Eagles (2nd place)
2009 - Storm (4th place)
6 teams have taken the week off and made the Grand Final, only to lose:
2001 - Eels (2nd place)
2002 - Warriors (1st place)
2003 - Roosters (2nd place)
2004 - Roosters (1st place)
2006 - Storm (1st place)
2007 - Sea Eagles (2nd place)
4 teams have won the title without getting the week off:
2002 - Roosters (4th place)
2004 - Bulldogs (2nd place)
2005 - Tigers (4th place)
2006 - Brisbane (3rd place)
4 teams have made the Grand Final without getting the week off, only to lose:
2000 - Roosters (2nd place)
2005 - Cowboys (5th place)
2008 - Melbourne (1st place)
2009 - Eels (8th place)
The Grand Final has been won:
3 times by a 1st placed side
5 times by a 2nd placed side
2 times by a 3rd placed side
3 times by a 4th placed side
Never by a 5th-8th placed side
The Grand Final has been lost:
4 times by a 1st placed side
4 times by a 2nd placed side
Once by a 3rd placed side
Never by a 4th placed side
Once by a 5th placed side
Never by a 6th or 7th placed side
Once by an 8th placed side
According to my handy numbers crunching machine, the best possible chance for Grand Final glory is to finish 4th (with a 3:0 ratio for 4th placed sides who make it to the big game), but it doesn't really make much difference if you get the week off or not.
What matters is that you turn up, play the game, and win the f**king thing.