I found that odd, as well. How can they afford the airfare all the time? I would expect Clark to be out in Kansas much more than the Kents would be going to Metropolis. Also, whose parents go visit them at work and become friends with their boss? If my parents came to town, the last place I would expect them to show up would be my office.
Well, Clark does spend a lot of time at the office, and his girlfriend is their, and he has a deep relationship with Perry.
Anyway maybe they first met Perry at the Daily Planet fair at the start of "The Source". That at least made sense because Martha wants to get Jonathan away from Kansas so he does not brood on the recent death of a relative, plus it is possible they came Superman express. At least it is not one of the times they explicitly come by normal means.
