The AVIC-D3 manual says connecting the reverse wire is important, but is it really if I am not using a reverse camera ?

No, unless you plan on driving a lot in reverse, in which I have heard it will throw off the navigation until it gets a new GPS fix.

no need if no camera, in fact if you hook it up you'll get that annoying caution message evey time you go in reverse

My idiot installer couldn't figure out how to properly install the reverse wire/signal, but the navigation in my 2008 Audi A4 still seems great. If you drive in reverse a lot, you could sometimes have problems while waiting for new GPS fixes, just as the previous poster claimed.