There aren't, but it's worth thinking about what a vote does. In Lemmy and similar systems, comments with higher scores are listed first and seen by more people. A downvote means "fewer people should see this".

Now perhaps you just believe fewer people should see things you disagree with. That's not my position, but I won't try to talk you out of it here. If that's not your position, however and you instead believe civil, well-reasoned discussion between people who have different viewpoints is valuable, then using votes to simply express agreement or disagreement will not serve to promote such discussion.

Definitely not a strict rule and I wouldn't want to force anyone to do it the way I do (maybe I should have marked my comment as a joke) but as far as I understand, downvotes were originally meant for spam or low-quality/low-effort comments. Stuff that just doesn't add anything to the duscussion and isn't worth reading. Fortunately, that doesn't happen very often on Lemmy.

Downvoting comments that you disagree with just to bury them, especially without even leaving a comment that explains why you disagree, just feels petty.

Overall, I'd rather upvote a well-written comment even if I disagree with its contents and downvote ten "yeah, same" comments that agree with me but add nothing to the discussion.

It's more like "etiquette." The downvote button serves to bury a comment or post, so it's arguably best to use it as a "this isn't relevant or is hateful/unproductive" button.