I have the book "Yeast". It's a tough read, but very valuable. I recommend it for anyone that harvests their yeast.
From what I've learned, I think that you get less mutation when you keep the wort oxygenated (as in yeast starters), than when you're having your yeast make beer. This is why I try to actually grow yeast out of the primary, so that I'm using yeast that are more pure and true to the original strain.
Basically, when your yeast are putting all of their energy into multiplying, instead of into making beer, they are less likely to mutate.
I did an English Bitter and harvested the London Ale Yeast out of it. I got three pint jars of yeast, each of suitable quantity for additional batches of beer. Instead of using them to make beer, I did a yeast starter with each. I turned each of those three into two new jars of similar yeast count. I basically had the equivalent of six batches of yeast out of one primary. That yeast is $6.99 per smack pack at my local homebrew supply store. That's $42 worth of yeast savings from one harvest. I can make a fairly high gravity beer with those savings! An if I harvest the yeast out of that batch, I have 6 more, bringing my supply of yeast up to 11 pint mason jars.
There is some costs though, as it requires an investment in DME of 2 cups per starter. So it's necessary to weigh the costs of the DME in the starter, compared to no cost for only harvesting out of the primary. DME is about $4 per pound at my local homebrew supply store. I can make quite a few starters at $4 per pound.