Depends. There are always number of sexual offences at the Octoberfest including a number of Rapes.
The estimated number of unknown cases accepted among experts is 10 to 20 times higher.
If your going to get completely inebriated, then yes, there is a higher risk of getting into trouble.
I Recommend you :
a.) Choose you time carefully. eg, go during the daytime or early evening
b.) Choose your tent carefully. If your going to the Hofbrau Tent and end up in the middle, then it can be very unpleasant. Go to shottenhammel for example, and it will be completely different.
c.) If you can, try and get to know some people who are going down so that you can be part of a group. (Keep an eye on TT, there is often posts asking if anyone wants to join up and go to the fest)
Still, considering the numbers of visitors, the percentages are very very low. But i have to say, if my partner was to go down to the Fest on the evening of the Second Saturday by herself, would i be concerned for her Safety? Yes, extremely.
Anyway, check the link for some tips and good luck