However, I do think there is a value in not having a too huge gap between how many people you seem to be able to potentially reach and how many people you can actually reach. Not so much if the Twitter accounts is your personal account, but I think it can be an issue if you run an organization's account and your boss thinks you can do much more than is actually realistic.

However that risk has to be weighted with the risk of blocking people accidentally because of false positives.

Personally I think it also depends a little bit on the account size. If you can easily cut a few thousand fake accounts from your follower-list without spending too much time on it, then why not do it on some slow day in the office.

I believe this kind of spam is mainly driven by auto-following-back, which should be discouraged and in my opinion betrays a poor understanding of the medium. Even if one takes care to organize the accounts one follows into lists, or by means of some other tool, what is to be gained by following tens of thousands of Twitter accounts? Mindless auto-following has spawned services like TrueTwit, which forces users to jump through hoops when following a Twitter user. This is just dumb! By now I think everyone knows that spam accounts may be among one's followers, and it should simply be agreed that it's not important.