What happens if Wonder Woman didn't deflect a bullet with her bracelets and if it hit her in her head or non-armoured part of her body, would the bullet penetrate her body?
She would be injured, but she does have superpowered healing abilities
During the first battle scene on Themyscira, Diana does sustain an injury/flesh wound on her left arm which the movie seems to imply was a deflected bullet.
This implies that the DCEU version of Wonder Woman, at least at this point in her timeline, can have her skin broken by bullets or kinds of trajectories (like shrapnel).
Later on, though, the healer removes her bandages and sees her skin has healed fully, surprising her
Since we don't see it happen in the movie, we can only speculate.
There is some evidence that being shot might hurt (but not penetrate) since she feels pain when training early in the movie and is, initially, forbidden edged weapons.
However, when Diana comes into her powers, it seems she obtains "god-level" power (or at least demi-god) and is probably invulnerable to pretty much everything....even if, at the time, she isn't aware of it.
Ares even says that..
"only a god can kill another god"
..so it seems likely that a bullet might sting but not penetrate and/or kill.
Using the bracelets is more of a "convenience" than a necessity.
In the comics her powers have fluctuated over the years to Superman level (and beyond). For more on that I suggest you see this Q&A over at Science Fiction & Fantasy Exchange