As many agents explain that negotiating is an art and takes skill, a lot has to do with timing. In sliding door moments, when to play hard ball can completely ruin your chances of ever purchasing.
When trying to play hard ball, what you need to realise is that the real estate agent is on your side. Whether you are the seller or purchaser the real estate agent is in between and 9 out of 10 times also wants to ensure all parties are happy. This is not received well when explained to purchasers as the initial reaction is that the real estate agent is only getting paid by the seller. True, however one must always remember that a purchaser will always be a seller in the future.
Playing hard ball will never help get you to where you need to be. I'm not saying that always listen to your real estate agent and I am definitely not saying to roll over and just show your cards. Its just always good to know that the real estate agent is just as a motivated to help both parties and not try to favour one.
In conclusion to the heading of when to play hard ball - never is the answer. If its only going to hinder to where you want to be than there is absolutely no point. Maybe being more transparent and open will not only save you time to do things you want but get you to where you need to be.