That's not always true.
For example, Buffalo has gone to Arizona for the last two times they have played. And the Bills haven't played in Seattle since they crushed them in 2004 (anyone remember the fake qb sneak by Bledsoe, then toss to McGahee for the TD? Wish they would run that again!). Last two games against them have been at home (well, one in Toronto last year, but you get the idea).