Your described alignment problem is not normal by any means. No car just "goes out of alignment" unless either something is worn (loose) or damaged from a huge pothole (not average potholes) or bumping the wheels into curbs, hard enough to bend parts. The potholes don't usually do more than bend aluminum wheels and Volvo mags bend very easily, hard to avoid sometimes. When a mag's bent, you'll get vibrations usually felt in the steering wheel if the wheel's in front.

I work for a large Volvo dealer with many years of experience, we have a new state of the art Hunter PC alignment machine and when it says the car's in alignment, there's no doubt about it. We don't see any Volvos or other makes returning for periodic alignments, doesn't happen so I'm thinking that either someone's overlooking something, not tightening adjustment bolts tight enough or your problem might then be tire related.

Despite the Sears, Firestone, Goodyear type alignment hype saying that cars can just "go out of alignment every .....miles" that simply doesn't really ever happen on any car from normal driving. Once a car's aligned, it stays that way unless damaged or parts wear out. The last time I've aligned my own cars was years ago and they still go straight (no pulling) and the tires wear evenly, last along time.

Seek out either another shop, preferably a good Volvo dealer with good equipment and insist that the car's checked by an experienced Volvo tech or it could be dispatched to an hourly "herman" (happens). Be sure to describe details of your past experiences to the writer as you shouldn't need to keep having this problem and it should be solvable if a tech knows of your problems and spends a bit more time double checking the car for something out of the ordinary.