Does Wonder Woman need to use her bracers to deflect attacks or could she simply withstand the attacks she normally deflects, e.g. bullets? Does she use the bracers just as a polite formality?
The characterization of Wonder Woman has varied widely over the decades since her creation. In the beginning of the character's existence, she was NOT bulletproof and her bracelets were necessary to deflect bullets.
Wonder Woman was created by the American psychologist and writer William Moulton Marston. The character first appeared in All Star Comics #8 in December 1941 and first cover-dated on Sensation Comics #1, January 1942. REF: Wikipedia - Wonder Woman
In her second comic cover appearance she was seen deflecting bullets fired from machine guns... Since she was not bullet-proof in 1941 she absolutely needed to deflect bullets with her bracelets. Reading through many of her early adventures, Wonder Woman seemed to always be using this particular power to defend innocents from enemy fire in addition to herself.
Note in this series of panels from a 1945 Wonder Woman Christmas tale from Sensation Comics #38! She deflects bullets to protect herself from being shot at. But later in the same page, a bomb is detonated and everyone except her is knocked down in the explosion. Her hands are tied behind her back with her lasso and she did not deflect anything. Even then she was a tough customer!
In the initial character creation, the bracelets were used to deflect bullets as the ultimate sign of superior martial skill and hand-eye coordination. "Bullets and Bracelets" became a signature feat in the minds of the public at large when Wonder Woman was portrayed by Lynda Carter on television in the mid 1970s.
Diana is depicted as a masterful athlete, acrobat, fighter and strategist, trained and experienced in many ancient and modern forms of armed and unarmed combat, including exclusively Amazonian martial arts. In some versions, her mother trained her, as Wonder Girl, for a future career as Wonder Woman.
From the beginning, she is portrayed as highly skilled in using her Amazon bracelets to stop bullets and in wielding her golden lasso. Batman once called her the "best melee fighter in the world". The modern version of the character is known to use lethal force when she deems it necessary. In The New 52 continuity, her superior combat skills are the product of her training with Ares in her childhood.
In her latest incarnations as an avatar/goddess of the Greek mythos in DC Comics New 52, able to fight Superman to a standstill, she may be simply tough enough to resist small arms fire outright.
However, her bracelets are god-forged weapons, nigh-indestructible and as a result can be used to deflect far more powerful attacks as well.
In Justice League Unlimited she gets turned into a pig and Batman and Zatanna have to save her and she gets caught in a butchery where pigs are slaughtered and almost gets shot by a guy and barely manages to lift her now pig like bracers up to stop the bullet.
They make it look as if she would have been shot in this form if she didn't use her bracers.