The weights won't un-tweak your car, but will improve the weight balance. Have a look in the FAQ, depending upon servos and radio gear, I think the weight varies from 60 to 100 grams to the left of the tank.
If you've got Ray Wood's Winternats DVD, check out Michael Salven's 710 - you can see an Xray weight bolted to the chassis by the tank.

To tweak your car, follow Julius' method in the FAQ in my signature - this is the best tweak method and the most accurate