Mark,
Unless you or your crew are on the short side to start with, we find it not an issue. We have the Nav Pod on the upper sloped section of the bars and can still easily see over it and I'm 5'-7" tall (I'm starting on the shrinking path

). To be honest we mostly sail standing or sitting to the side of the helm. I find I'm primarily directly behind the wheel only in rough weather, in the marina or anchoring and when I want to look at the plotter.
When we bought our boat, the plotter was down below at the Nav Station, and it was a pain. Someone was always going down below to check something. One moonless night we were coming into an anchorage and my spouse was down below broadcasting up what I should be seeing in the dark. Not a very comfy feeling. I then ended up moving the plotter to the helm and installed a black & white repeater monitor down below.
Roc's idea is a good idea to get a feel for if its going to obstruct your viewing, but the plotter really should be readily available for the helms person.
IMHO.