NFL Power Rankings do not matter, particularly after the first week. While it is fun to try and rank teams in preseason, ultimately, contests are decided on the field. A team must play its schedule, and a champion will be crowned after the Super Bowl. Rankings that are created by people and computer programs are unnecessary.
The NFL power rankings can change, and they matter to a certain extent, but don't account for injuries, suspensions (Tom Brady) etc. I think they matter more later in the season, since this early on, no one really knows how teams stand. Sometimes it takes multiple games for a team to gel and reach its potential.
No, NFL Power Rankings do not matter after week 1 because there are so many games left to be played. Teams will look completely different by the end of the season. Power Rankings only matter before the playoffs. A team could face injuries that will leave them behind, despite a strong week one.
Power rankings are simply an excuse for talking heads to chat about football. Of course the rankings don't actually mean anything. You don't get an advantage in the game if you are ranked higher, and the teams themselves don't care a whit. However, ESPN, NFL Network, FOX1, all these stations love the power rankings because it gives them something to fill up that long 24-hour day.