It's hard to know. I generally try "well enough" and provide a couple of incorrect positives and negatives. I was forced to do about 8 yesterday, and I messed up all 8 slightly. It might depend how overtly you try to game it. I've tried to be subtle, but I don't know whether or not it's effective.

Back in the days of book digitising reCaptcha it was fairly easy to distinguish which word was from the book vs computer generated and only answer half the captcha and pass.

I always inputted the optional word with slight errors. A totally wrong answer would be easy to filter out, but one or two letters being wrong while it's still a proper word would hopefully be harder to get out of the training data.