Here are my thoughts on the subject of provoking. When we go to a client's location, we claim to be there to assist them in finding out what the cause is for the events that they have been experiencing and are supposed to be genuinely concerned for their safety and well being. At the same time we are trying to assist whenever possible those on the other side. We do this by researching the reports of activity through an assortment of verifiable records that may point us to the possible identity of the spirit and a plausible reason for the haunting.
If the haunting is determined or believed to be residual (an imprint on the atmosphere of the location), provoking will have no effect at all because the imprint does not react. If the haunting is determined or believed to be intelligent however, provoking may induce negative reactions and the "Ghost Hunter" may get his or her desired evidence..but where does this leave the client when the investigators pack up and leave? It leaves the client with an angry spirit and the likelyhood that the client's troubles are just beginning. This doesn't sound like the service we claim to be trying to deliver.
It has been my experience that if we really try to educate the client prior to,during and after the investigation about the possible causes natural and otherwise, we are empowering them to better understand what is happening and the reason for it. I refuse to believe that any good can come out of provoking a spirit. I would suggest using "trigger" items such as music from the period of the possible spirit, personal belongings or in the case of a child spirit maybe a toy and try to invoke some friendly interaction. By doing this the Investigator may get the desired evidence and the client now having a better understanding of their experiences will be comforted by that knowledge.
In the case of a demonic haunting, nothing could be worse than provoking for obvious reasons.
If the investigator isn't qualified (and most aren't) to address this situation, the client should be referred to a member of the clergy from whatever denomination they follow. Also do a follow up to find out how the client and family are coping and if the situation has been resolved.
Lets be professional out there people! You are doing this to help people, not to make it worse!