It really depends on who your users are, I guess. The general public aren't particularly grateful as a whole, but niche users tend to be more understanding.
There's also the case of implausible ignorance: the "I don't recall" defense popular in politics and business PR-speak. This tends to decrease credibility.
If you're not sure about a particular fact, make an educated guess if you can or relate what you can from memory, and say, "I'll have to check on that and get back to you," then actually check on it and get back to your questioner or the audience later.