I wouldn't have even thought of voting for my own pic, regardless of the rules. If I was lucky enough to get into the final 20, which I thankfully did, I'd vote for the best of the other entries.
When voting on this weeks challenge, I didn't even look to see how people were voting before I made my choice, so as not to be influenced.
I was stuck between 2 pictures, and seeing how everyone else was voting may have had a bearing on my final choice. It's human nature to gravitate to the majority opinion on a subconscious level, no matter how much of a rebel you think you may be.
I think it's a good idea to stop people voting on their own entry. It's the way in other photographic competition I enter. They also have an anonymous picture entry's till after the voting, so pictures are judged on the quality of the image, rather than the authors identity having any sway. Hard to do here, but the best way for an image to be judged only on it's own merits.
Also if people are taking the time to put pictures in to be judged, and others are taking the time to look, and vote, the least you can do is vote yourself if you're in the last 20. It's the decent and respectful thing to do, whatever the outcome you vote may have on your final placing. Be honest and play the game in the right spirit.
That's just my opinion, feel free to ignore.