How about: you never help anyone in the method you planned to. If you find that acting in your own self-interest helps more than activism, and that earning to give helps more than acting in your own self-interest, then I guess you wouldn't need to worry about it so much.
That is, you wouldn't need to worry about it when considering going into activism. You still need to worry about it when considering how to avoid backsliding, because not backsliding is still much better than backsliding.
There are diseased fields with net negative social balance (e.g. selling meth), but EA types probably wont' stray into these areas anyway.
It has been considered here: http://felicifia.org/viewtopic.php?f=23&t=472&p=3789
When I talk to people about earning to give, it's common to hear worries about "backsliding". Yes, you say you're going to go make a lot of money and donate it, but once you're surrounded by rich coworkers spending heavily on cars, clothes, and nights out, will you follow through? Working at a greedy company in a selfishness-promoting culture you could easily become corrupted and lose initial values and motivation.
First off, this is a totally reasonable concern. People do change, and we are pulled towards thinking like the people around us. I see two main ways of working against this:
One implication of the "won't you drift away" objection, however, is often that if instead of going into earning to give you become an activist then you'll remain true to your values. I'm not so sure about this: many people who are really into activism and radical change in their 20s have become much less ambitious and idealistic by their 30s. You can call it "burning out" or "selling out" but decreasing idealism with age is very common. This doesn't mean people earning to give don't have to worry about losing their motivation—in fact it points the opposite way—but this isn't a danger unique to the "go work at something lucrative" approach. Trying honestly to do the most good possible is far from the default in our society, and wherever you are there's going to be pressure to do the easy thing, the normal thing, and stop putting so much effort into altruism.