That's why I insisted on highlights instead of a full-head dye. My hair grows- as the hairdresser put it- like a weed. So the chunks were perpetually different lengths of grown out so I didn't have a solid dye-line. As far as the last round- I got pissed off, dyed it blue, and when it started to go green I dyed it brown so it wouldn't be terrifically obvious that I was growing it out. I'm not sure how that would work on blond hair, though. (hmm- I really need to re-dye the top of my blond streak. At some point)
I think if you keep it subtle and the chunks a bit uneven, it could be done on an "as able" basis.
It is unfortunate that ass or moron are the choices.