Ah, now I'm not suggesting that you go in entirely unprepared, rather, you shouldn't water down your approach to do so.
Basically, if you throw your lot in and try for the 'I'm ready for it all' approach you'll be letting the list down somewhere.
Make provision for a core in an escalaltion scenario (My list doesn't really I know) but dont go all out, or you'll be changing your list to something you'll be less happy and familiar with.
Competetive as a GT is, I still think that tha aim of the day is to have as much fun as possible and try to bag a prize or two, but if you lose? well, in the GT there is only one winner, so there are about 199 losers in total... it's no big deal if you are one of the 199, might as well go and have a laugh, as there is a good chance that you may be in that group.
Not saying don't go to try to win, obviously not, but the game is for fun, and you cant win them all, try as you might. Might be worth taking a view perhaps to see what scenario you think you can drop from your preparations (due to theme/tastes/style) and focus up on the others. It's a negative view true, but like I said, you can't win them all.
(It's at this point I will now mention that I came joint second in the first Call To Arms Tournament for Mordheim and I had a list that was totally themed, and really not very tactical, and though I didn't win 1st place I was one Ld test away from the difference between 1st and 2nd, and I didn't mind failing it that much because the whole competition was brilliant fun!)
Indra