Centering

[ls7plus]

One of the nicest things about this hobby--aside from all of the very knowledgable, helpful persons who are members here (you can all take a bow guys!)--is that you can collect what you like. Personally, I primarily collect stars and hall-of-famers, and agree that eye-appeal is very, very important. However, if you're after some really rare cards of key players, you may have to take what you can get, and upgrade later, when and if the opportunity presents itself. Again, it depends entirely upon what you like!
From time to time, I find myself pursuing "true" rookie cards of hall-of-famers, such as the 1947 Tip Top Bread Spahn, Kiner and Berra. It took me about ten years to find a Kiner offered alone, rather than in a large lot at auction. I almost passed on a poor example that had been written on several years ago, reconsidered, and went ahead and purchased the card. I have always been glad I did, since the only one I have seen offered alone since was another PSA 1, which wasn't really much better than my example! I also bought a Spahn graded PSA 1 MK after almost passing on it due to condition. Then I checked the PSA pop report and found that PSA had only graded six examples (which hasn't changed in quite awhile), with the highest being PSA 1! Another member of this board recently commented that the Spahn is the toughest star card to obtain from the set, as it is a short print in a set that is basically pretty rare itself.
Similarly, I recently purchased a 1956 Kahn's Frank Robinson, after passing on one with back damage more than a decade ago. Interestingly, the card I passed on (then offered by Larry Fritsch) may well have been the very same one that recently became available to me, as it also had some very minor back damage, but otherwise presented as EX+ (was probably glued in a scrapbook, which left the rest of the card pretty well preserved).
Centering is important, but if you're after a very rare example of a card you've been looking for quite awhile, I personally would buy first and look to possibly upgrade later!