I guess it depends on what you are playing. If you are doing a Hezekiah Walker song, you probably wont find the horn parts written out, so you will have to learn them by ear, or get with the other horn players and figure out the parts. You can also get a little music notebook and write out the notation once you figure it out.
If you are doing hymns, then you can sight read from the hymnal as long as you know how to transpose it so that it matches your horn, and you know what key they are singing it in.
IMO, I think its way easier to just do things by ear when in the service...you cant really flow when you are stuck on charts and sheet music