While Rock is statically proven the be the most frequently chosen hand, it's also accurate to say that many people, knowing that, usually start off with Paper. Based on this logic, I like to go with Scissors a couple times to bait the Rock, then use the Paper accordingly.
Of course, since me and my friends use RPS (best of three, though) as a constant way to decide on stuff, we are so used to eachother's patterns that there's a need to trick eachother with mindgames. They know that I usually start with Scissor, but they also know that I know that they think that and use Paper accordingly. This is where Rock comes in- it serves as an unexpected but straightforward offense, usually opening for a Scissor-Paper interchange later on.