As many often point out; Bill and Ted only learn Rufus's name from their future selves; who heard it from their future selves; etc. The name comes out of nowhere. However, in order to keep the timeline that he wants to create; and because it's rather an honor to be given a name by "the Two Great Ones, he went Sure, Let's Go with That. * Also, quite a bit of Smiling And Nodding is required with Time Travel; to avoid Paradox.