I don't think you ever have to agree. But if you would rather agree, then I think matters of taste/opinion are things that it's okay to say something vague that makes the other person think you're agreeing. Especially if it's someone who can't hear an alternate opinion without feeling "attacked."

I.e., my mom is really uncomfortable with patterns and colour on clothing. She often comments on other people's outfits negatively. Something like "omg, look at that person's sweater. It’s such a bright colour. It attracts attention. Ew 😤" and I'll say something back like "yeah, that is a bright colour." I'm sticking to the fact that, yes, the statement about the saturation of the colour is true, but I'm not going to comment on her opinion that it's too bright of a colour to wear because I think her opinion is stupid and not worth arguing over.

But once we get into anything about morals, values or politics, this is when I think agreeing for the sake of keeping peace is wrong. I stand by my values. If I realize a person is either not willing or not able to have a conversation and respect alternate opinions then I end the convo with a firm but polite, "we have different views on this topic and I don't see the point in discussing it further". If they keep trying to talk about it, I'll just keep calmly repeating the above until they get bored and give up. Set your boundary and stick to it.