We bumped into Cinderella a number of times on our last trip....
Once at the DL entrance, along with the rest of her story characters.
Twice at Princess Fantasy Faire.
Twice at the short musical animation show that they put on in the Hollywood backlot stage over in DCA (by Monsters Inc). I think it was called Drawn to the Magic, and there were always 3 characters and afterwards they'd do meet n greets - we watched it twice and both times the princess part of the show was Cinderella. The line to meet Cinderella afterwards was relatively short (5-10 minutes) and she spent some time chatting with our girls and posed for several pics in different poses....very low key and nice way to visit with Cinderella. Both times Cinderella was on the left side of the stage so sitting towards the left front would put you in a good position to meet her right as she comes off the stage. (we also saw sorcerer mickey both times but I can't remember who the third was but both times it was someone we'd already visited - maybe Woody I'm thinking) It plays several times a day and both times we just happened upon in when riding Monsters Inc. On another site, it says that Cinderella is often there.