Well, I think it depends upon you and the guy. I like to ease into things, and chat for a while first, but I quite understand it. It's both a cue that you're ready for action and a safety valve as NYCe said regarding LE (law enforcement), since they are going to have a hard time busting you if they're already naked. I don't think you need to jump right to "why don't you get comfortable," but don't start anything until you do. Always invite the client to get comfortable before you begin the session in ernest, even if you've been sitting sharing some wine for a few minutes of small talk first.