Where are you located? I'm in Baltimore. MD and here you need a filter on computers if you have X10, otherwise anywhere you have a computer, X10 won't work on any circuit that has a computer on it.
I also live in MD (in the DC suburbs), and while some of my PCs are filtered, others are not. However, with my XTB-IIR, I might not even need the filters I have (but they were in place before I got the XTB-IIR).
As Dave said, it really isn't a matter of location. It is a matter of how noisy (or signal-sucky) the power supplies in your computers (or other electronics) are.
True, I'm in PEPCO's service area, but I don't think BG&E is is doing anything special that causes EVERY computer power supply in Baltimore to generate high amounts of noise. I think you just happen to have been "lucky" with the power supplies in the computers you bought.