Even during the voting it should be ok to develop your game further, btw. As long as you do so in a seperate branch without modifying any of the binaries (executables) on your project page. As long as judging is in progress people should (be able to) judge the game based on the version of the moment of the deadline.
During the voting period only bug fixes are ok, especially bugs that make your game unplayable. Major game play changes or extra polish are not ok during the voting period. Or you could create a fork call post-jam, so people know now to rate that. Like LucKey said, after voting is over your're free to do whatever you want with your game.