I just purchased a 9mm shield and I also am trying to decide on a carry ammo. S & W seems to issue a disclaimer regarding the +P ammo. I suspect that should you have a problem, this is what they will point you to when they discover you have been using +P ammo.
With regard to +P+ ammo, they come right out and say it must not be used.
As a matter of interest, they are not the only gun manufacturer that holds these positions. It seems apparent that when you use +P ammo, you do so at your own risk. Also, this is something one must consider when deciding whether to buy a new or used pistol.