The witness can change the code they are running at any time, just as you can change your vote. IMO unless you plan to monitor every situation in real time, you are much better off voting for witnesses whose judgement you trust as that is something not likely to change from minute to minute. Of course you are free to vote however you like for whatever reason you like, that was just my opinion.

As far as whether your votes matter, all votes matter. There are many more people with smaller stake, so it adds up. Only by not participating do you guarantee that your vote doesn't matter at all.