I think there were a few incidents last year that created a fair amount of concern among female solo peregrinas. Falcon's link to the statistics reinforces the bottom line that Portugal is much much safer than the US (don't know about other places), especially when it comes to violent crimes.
I have walked thousands of kilometers alone over the past 11 years, and have been on some pretty remote Caminos, and I have never felt physically threatened. I have seen "flashers" about 4 or 5 times over all those caminos, usually when I was in the company of another woman, never when I was with a man, and occasionally when I was alone. I'll admit it freaked me out a bit, but I kept telling myself that people wiser than I always say that exhibitionists are not likely to be involved in a physical sexual assault. There is petty theft, of course, mostly in the big capital cities.
I think the bottom line is that you should of course be careful and alert, but for the most part you shouldn't worry about your physical safety even while walking alone. As Portia points out, it may not be a bad idea to have a buddy in case you are injured, but it's probably not necessary for crime prevention reasons.
Buen camino, Laurie