I get an allignment:

once at the beginning of summer (for autox season)
once at the end of summer (for winter driving--raised hight)
whenever I do any suspension work, since that tends to screw up your allignment.

Checking tire wear isn't big for me because my tires wear wierd anyway ( I run 2.5- camber f/ 1.5r in the summer), plus I do susp. work often enough that I get allingments faster than the tires wear. For a normal car though, checking tire wear is the best indicator.