Obviously it is okay to downvote. However, how you do so affects others and your own reputation. What's more, it can have a longer effect on your own time and engagement with SO. Regarding responses from others, there are normal people, and there are folks who go on a downvoting spree after a negative comment or a downvote. That response recently happened to me. Those people are nutcases. The question is, do you want to look (1) helpful or (2) strict, and do you want to risk provoking a nutter? Or do you want to do downvoting and commenting in a mutually exclusive manner, so that your downvotes are anonymous?
I think a helpful comment and a downvote, followed by an appropriate response when the answer has changed (i.e. removing the downvote or giving an upvote) are particularly useful. I will sometimes edit the answer or leave a suggestion in the comments, rather than downvote. This is because I don't tend to revisit answers - if the person fixes their answer, good. If not, well, my comment is there. I am more likely to revisit questions.
The unfortunate aspect is that there are a few nutters on the site, so a comment or even a vote to close may provoke them to become vengeful downvoters. In those cases, it's best not to respond in kind (not that you can predict these people in advance). The advice I've read is to alert the SO team or a moderator. Still, having inadvertently provoked a couple of nutcase, I wonder about whether or not it's best to leave a comment and downvote.
I'm leaning toward mutually exclusive downvotes and comments, simply because there's little to be said for dealing with the demographic that can't handle criticism. SO is not like, say, academic teaching, where you get a few mediocre students who become fussy for bad grades: online, these people become nasty stalkers.