Most people will say the shorter the better. There many factors that come into play but in flat water riding, on most AM hulls you want to get your weight as far forward as possible. Also, in general, the taller you are the shorter your pole should be. I think stock length is around 40-41 inches. I have an xmetal pole that is -5 from stock also running a shorter turn plate and straight bars so probably closer to -7-8. I'm 6'4" and wish mine was maybe a few more inches shorter. My son is 5'8" and he even would like it a little shorter. It does come down to personal preference. Take your ski out and ride it for a while. If it feels like it's riding nose high all the time, then shorten the pole a few inches and repeat. I'd ride if for at least a tank of gas or the day before shortening. In my experience there is a point were if you go too short, it may make doing some trick easier but it's hell on the back from being in a bent over position all the time.
There's lost of discussions on this site about pole length if you search for them.
Here's a good one that's talking about xmetal poles but it has a few posts listing actual lengths in comparison.
http://www.x-h2o.com/index.php?threads/chan-pole-length.158488/