Yeah, it definitely decreases the chance of pregnancy, but is one of the least reliable methods. I wouldn't trust it, no way!
The anatomy/physiology of it is enough to warrent a back up method, and not to mention that it does NOTHING to protect man or woman from sexually transmitted diseases. So I'll have to be the odd man out and say, no, it's not safe.
And, uh, it can be kinda hard to practice sometimes. If ya know what I mean.