Again, I can only speak in-depth for Catholic churches, though I've heard stories of others that suggest it probably holds true for them, too.
But the nice thing is that many Catholic churches in my area have some of this spelled out in handouts for couples looking to get married. Just ask at the rectory... That and the church itself is generally open during the day for visits. Be respectful (I'm not worried about you, Bob, but who knows who else will read this), and you can almost certainly at least go in and do a recon walk through with a light meter and to get ideas ahead of time. The odds are pretty good that you'll even be able to take some sort of set up or trial pictures, especially if you offer to share them with the parish...