Good question! I've been wondering the same thing. I haven't been using clearomizers for all that long, and I've been curious about that.
Personally, I think that I'm kind of over-the-top with my coil/tank cleaning, and I throw out the last 0.6-0.8 mL of a full tank about half the time. If it's a lighter liquid, I'll top it off a few times before I go for the bottom. The only juice I have that I'm comfortable going for a few tanks in a row is MBV's Berry Creamy. I like a lot of darker ones, and they don't fair that well being re-filled a few times. Especially Halo's Cafe Mocha. That's my favorite flavor, but it starts to look like motor oil faster than my others.
But I'll also say this: I've never had a problem topping off over and over. I just don't like the looks of it, and it seems like it's begging to cause a problem.