For some reason I find it annoying when people do that. For example I often hear something like ("I don't know but, the library you are using is bloated compared to library X").
I frequently catch myself saying "I don't know" in front of assertions I make. This is an issue when talking to a less knowledgeable person, because the "I don't know" puts the more knowledgeable person at an immediate disadvantage, and lets the person with less knowledge have more of a say in how things should be run.
I find better just to confidently state something, and get called out for inaccuracies (it's certainly more memorable). Also think about what you said at the end of the day. It's better to recognize and admit inaccuracies than to hang onto your pride (http://www.google.com/search?sourceid=chrome&ie=UTF-8...).