Alignment Question

Ok, not being a mechanic I don't know this answer so bear with me. My question is do all cars need a "4 wheel alignment"? The dealer across the street says they do. I asked why? I replaced the struts in my car (with the help of a friend) and as far as I can tell, there's 3 bolts on top, the big mother at the bottom, and a couple little ones holding the brake lines & the stabilizer bar. There's no room for movement left, right, sideways, up or down...they bolt in one way, and one way only. He tells me there are "shims" and "YOU won't see them" (in a negatively conotated sort of way - like I'm a complete idiot, and maybe I am - he could really take a course on how to deal with people). "The wheels have to come off" etc.
So, they (these shims) would't have fallen out when we did the struts? I mean the wheels had to come off for that too. The only reason I was asking him was because I wanted to know what a 2 wheel alignment cost since it would appear to this 'idiot' that my car doesn't need all 4 done. The back only follow along one way and I didn't see any way at all you could adjust them for camber or whatever. Nor did my friend and he is fairly mechanically inclined.

Just wondering...

Thanks.