Response from ensoul <Lensoul99@aol.com>:

My spouse does it whenever kitty is being mean for no reason.  
Sometimes it is a little late in my opinion if kitty hisses, you snag
the water bottle and squirt, it is a good five seconds at least from
the time kitty hissed.  So I think if you are able to do it quickly,
spray whenever.

Otherwise, only when they are fighting.

And to be honest, I am not entirely sure spraying a cat with water is
OK.

Not I.

My opinion is, *never* use the squirt gun.  The cat would not be hurt
physically, but your cat may learn to associate the action only with *you.*
One person told me about how she thought here cat had been taught not to use
the counter -- but, on one occasion when she arrived home from work, she
happened to glance at the window and saw her cat on the counter.  When she
opened the door, the cat was in the other room and "appeared" to be resting
on the sofa.  It is much more effective to use some form of positive
distraction.  For example, wave a wand with a feather on it or simply call
the cat to you.

MaryL