I recently heard that one is forbidden to get any benefit from a church (or any other place of worship other then a shul). They went on to say that the prohibation included while giving directions "turn right at the church". Has anyone else heard of this prohibation being that extreme and what sources that?
Please ask your rabbi; I've heard of this practice as well; if it's actually a law (and not just a recommendation), there are two sources that I know of, and I suspect it's a combination of them. I hope this provides a good starting point.
A.) Saying the name of a heathen deity
Talmud Sanhedrin 63a (also Rashi on Chumash):
The Rambam rules (Avoda Zara 5:14) that any idol mentioned in Tanach is okay to say. Only other names are prohibited.
I suspect this is the source. The Gemara above would seem to be stating it's a problem when naming the specific deity; I'm not sure how that translates into "turn right at the church on Main St.", where no deity is mentioned.
B.) Deriving benefit from an idol
We're prohibited from deriving tangible benefit from an idol. A building that was built to house and protect the idol would be prohibited as well.
To extend this prohibition to saying "turn right at the church" , you'd have to argue that:
It could also be that for some of the reasons that it's a problem to enter a church (looks wrong, bad influence, etc.), it's recommended not to signpost them, but I'd have to see a source for this.
Many years ago I wrote this question in to an author of a sefer on Hilchos Akum. I argued that it should be permitted based on the reasoning Shalom provided in his excellent answer.
The Rabbi responded that although my argument seems correct, since there is a minhag to refrain from using Avoda Zara in this manner, it should be avoided. He added that in general, the practice of Jews throughout time was to steer clear from anything that smacked of Avoda Zara.
(To illustrate, A relative of mine who lived long ago in Europe told me that the practice back then when passing a church was to cross the street and refrain from looking at it.)
I stand corrected by Shalom, below, that this prohibition is only when mentioning the Avodah Zarah's name.