I have always thought that you could answer "it's ok with me" or "it's fine with me" when you agree with something that somebody proposed, like a meeting time.
But apparently the phrase can have a negative connotation.
According to this Urban Dictionary it means:
I could care less. Usually used when someone is being overly dramatic and is explaining the extreme action they are planning to do in response to a situation that they think you should care a lot about, but still somehow you don't similar to "that's just fine with me"
Since I wasn't sure if this source was reliable, I googled it and apparently the definition is correct:
"We made our play, and I came out on top. Okay? Now, if you want to start the game up again, that's fine with me." The Italian Job (film)
"I'd never have a hookup but if that's what someone is into, fine with me." Comment on a blog post
Can I be misinterpreted when I say "it's fine with me" meaning that I agree with the proposal? My concern is that it can be understood as "ok, i don't really care."