I have Google Voice and my Sprint phone integrated at the Sprint account level. It's been that way for about 4-5 months (whenever they first started offering it).
In that time, virtually all of my outgoing usage has been counted as a mobile-to-mobile call, even if the call terminates to a landline number. I don't know if this is by design, if it's permanent, or I'm just a very unusual (and very lucky) case.
My suggestion, is to make a couple of short calls, preferably to your conference call access number. Then log into your Sprint account a day or two later. See how that usage affects your available minutes.
As new as it is, I'm not really sure you'll find someone at Sprint and/or Google, who can give you a straight answer. We're pretty much on our own, when it comes to discovering the quirks of the service.