Yeast Washing: How big is too big?

I just began harvesting and washing yeast to build up my yeast bank. It has been said that you might not want to harvest yeast from a really big beer.....I'm guessing this is because these yeast are stressed and not as healthy. So, how big is too big?

I have a stout with an OG of 1.080 that I would like to harvest from. I pitched a properly calculated starter on this beer and fermentation has been lovely. Am I ok to save some of this yeast for the bank? How big is too big?

Thanks.

For me, it's anything above 1.050. But, that's just an arbitrary number that I use. I would think that 1.080 is pushing it though. Yeast isn't that expensive, when compared to a ruined batch.