That's pretty much my stance on it.
I'm competent at what I do as a DPS, so I see no reason to offer a disclaimer when I go to punch the hell out of things.
My adventures in tanking are another story; since I'm still new to it (half a level to go to PLD), I'll just offer a quick, "Hey all, just letting you know I haven't tanked this before. Please bear with me," and thank them for doing so at the end of the instance if they stick around. New tanks can cause some (or a lot of) frustration, so I figure it's only polite.
What Warren said also rings true, though. Being a liability to your party makes things harder for everyone.