Once again, I think I hop between both philosophies.
I don't always think it's appropriate to state my honest feelings/opinion about 'trivial' things, that aren't important in the big picture. And, if I don't think anything good will come, in the longterm, out of my saying something, then I won't say it. If I see no positive, self-growth 'point' in saying something, then I won't.
In everyday interactions, with more casual acquaintances, I think it's appropriate to be more 'civil' and not necessarily spew out all of your honest feelings about everything/everyone, to everyone. I don't really
see the overall constructiveness of doing that. For example, telling an artist their painting is bad -- I highly doubt the artist would really take that comment to heart and change professions - they'd probably simply hate the deliverer of the comment and continue with their bad paintings.

So what's the 'point' of delivering the more negative comment? Now as for me personally, if I was asked a subjective comment on a painting, I might say: 'Well, it's not my personal taste, but I'm sure a lot of people out there like it.' But I certainly wouldn't lie and gush with fakeness and say how much I loved the painting...because it wouldn't be true.
And yes, I do vary my approach depending on who I'm with. Some people I've encountered aren't open to any sort of honest feedback - they have their blinders on - so, there's not much 'point' in my saying something. Other times, people may be more open. I also modify my approach based on person, because I know people operate in different ways, and being a sensitive person myself, I'm sensitive to others who might be this way, and who might integrate a flippant comment a LOT more, than, say, your strong T people. That's more the empathy thing I suppose...as a child I was teased and I really integrated that and hence as an adult I'm very careful not to be 'mean' to anyone, because I know what it feels like on the receiving end.
But I definitely believe in honesty for the big stuff, like keeping relationships healthy. I don't think a healthy relationship is one where one person is constantly biting his/her tongue, or repressing part of himself, simply for the sake of getting along -- that wouldn't be a friendship, in my mind.
As usual, I think there's a balance between the thinking and feeling.
Eh..substitute, I agree with your idea that some people just inherantly may not get along, and don't mesh well. I experienced this quite recently with someone. She and I had been friends for about 2 yrs, but by the end of the 2 yrs it had become apparent to me that we were close friends simply because I was holding much of myself back. I'm not going to go into it here, but yep, we definitely were too different from each other for it to really work. My friend didn't want to hear ANY of my feedback, and thought the fact that I might disagree with her meant I wasn't a 'true friend' and wasn't supportive...so, the friendship ended. Because it wasn't working for me either.