Comment upvotes are one of the least important parts of SE's functionality, so officially it doesn't matter - you definitely won't get in any kind of trouble for upvoting these kinds of comments on your own posts. And your conscience is your own affair: you'll need to decide for yourself whether or not you'd feel bad about upvoting such comments.
What is the primary implication of upvoting a positive comment in this way - agreement, or acknowledgement?
In general, there are many different reasons for upvoting a comment:
- It's useful. If a comment adds something useful to the post, or makes a good suggestion for how to improve the post, then it's likely to get upvotes for that reason.
- People like or agree with it. Even if it doesn't actually help improve the post, a comment making a point which people agree with, or a joke which amuses people, will often get upvotes.
- For visibility. In a long comment thread, the default is to show only the most upvoted comments. I'll often upvote a comment which is important for some reason just so that it's seen.
Don't overthink it. And remember, comment upvotes are still anonymous, so even if you do upvote a complimentary comment on your own post, nobody will be able to think badly of you for it, because they won't know it was you who upvoted!