I read on this page,
Previous Fermentation: Always harvest from a low gravity and low hopped beer. High gravity and/or highly hopped beers can stress the yeast and have detrimental affects on future fermentations. Do not harvest yeast from beers with alcohol contents greater than 6.5% ABV.
My beers I have been brewing since I started washing yeast have ranged from 4.5%ABV all the way to 10% and lightly to highly hopped and I've harvested yeast from all of them. Because there are varying yeast strains in each batch, I was planning on only reusing them for similar batches, but since they don't really explain why (other than saying it "can stress the yeast and have detrimental *affects
", I'd like to know the reasons why and if this really matters as much to a home brewer or this is more of a guideline for a commercial brewer.
That should be *effects, by the way... sorry grammar police are out!