Most of the bikes I own would've been frame/fork-only and the parts of the day chosen to build them up. This doesn't really apply for me. However, when I first got into this craziness, matching date codes/PC components was all-consuming. I'm a lot more laid back about it these days (forex: my '73 Colnago Super has a '78 Campy SR derailleur shifting the cogs for me). I've come to realize it's just not that big a deal, plus from the saddle it doesn't matter a whit.
I do, however, draw the line at riding non-matching wheelsets. I'm OCD like that
DD