Gary, I would think that there is a bit of a difference between straight/curved roads and straight/curved swords-- a perfectly straight road has probably never existed, unless you want to count airplane runways. Swords on the other hand have come in a variety of forms, both straight and curved. The curvature (or lack thereof) of a sword makes a huge difference in how it is used, and the differences between such swords is an area studied by researchers. Also, whether or not the ninja used a special type of straight sword is an issue that has been hotly debated. So when someone on one of these forums refers to a ninja sword as being "straight," his statement comes with a whole load of implications that aren't there when an archaeologist says, "the Romans built straight roads."
David Sims
"Cuius testiculos habes, habeas cardia et cerebellum." - Terry Pratchet
My opinion is, in all likelihood, worth exactly what you are paying for it.