"The Boxster has negative camber on its wheels. This means that the tops of the wheels slant in toward the middle of the car. This negative camber will result in more wear on the inside edges of your tires than on the outside."
As quoted from:
http://www.986faq.com/2-0/default.asp#007
That wear pattern sounds normal, but without seeing your tires, its hard to say if you need an alignment or not. You may not, especially if you aren't having any driveability issues.
HTH,
Larez2