If Rob is with Nikki Reed he sure cheats on her all the time. Nikki was getting super bad press partying with him which is why they don't want to be seen together.
My friend has seen him leave parties with various models in the past few months obviously going home with them. The friend isn't a Twilight fan, or RPattz fan, so he'd have no reason to lie. Also, Rob clearly likes models so it makes sense as his fiancee was a model, he was staring at Heidi on Letterman, and Tyra on her show.
As much as everyone wants to link him to someone he is single. He is acting single. Let the guy have his fun while he is young.