I don't think there's really a concrete answer for your question (big surprise, I know). It definitely depends on the type of yeast, and especially on the previous fermentation conditions.
For example, I wouldn't rush to re-use a yeast that had just been used in a OG 1.101 Russian Imperial Stout, for example, whether it was 1st generation, 3rd, 5th, etc.
I also wouldn't re-use yeast that I had dry-hopped on top of it with 6 ozs of hops... unless, maybe, you're using it for another really hoppy beer.
But, I'm sure others disagree; you can have lots of different results with different situations. But I have reused yeast a couple of times in cases where I probably shouldn't have, and the resulting beer was sub-par each time. In cases similar to those mentioned above, I'm now at the point where I'd rather just buy another pack of Wyeast instead of increasing the risk of a bad batch.