The way it was explained to me at work was that the bottom of the side plates should be level with the bottom of the front and rear plates at the lowest (duh), but if you desire some more protection for your heart and lower lungs it's quite alright to hike them up a bit, and I know a few guys who do.
When SOP dictates that I must wear side plates I keep the bottom of them level with the bottom of my front and rear for the sake of mobility and comfort. I am issued 6x8's so I have a wee bit more coverage higher up than those running 6x6's.
If you're extremely concerned with side shots I have seen shoulder plates before, but personally I tend to gravitate towards less armor and more mobility. With my job I'm in a truck most of the time anyway, and if I'm not I'm usually hiking up to some ridiculous hilltop to fire a Javelin anyway so I pretty much run as little armor as they'll let me.