Thanks @jcobs! It's up to the bot owner whether or not the bot should comment on posts it upvotes. I may add an option in the software to pay a little extra for a "stealth" vote where there is no comment.
Also planning to add an indicator to the bot tracker site to see which bots comment and which don't.
Have to say that it's a bit demoralising to go down the comments of a post and see bot messages. But it's also good to know that the post has been artificially upvoted by bots. So it's 6 of one and half a dozen of the other :)