As a worship band it is often easy to overlook the fact that the congregation has to be able to sing the songs too. This means (at least according to my research) that you should stick to D (maybe Eb) to upper A for the melody line. Obviously there are exceptions, but this could be used as a rule of thumb.
As musicians the worship team normally enjoys playing the more difficult songs (both in terms of tonal range but also in terms of rhythm – syncopated beats being one of the issues) Modern worship music seems to have this very problem – the songs are quite complex and the tonal range of the melody is often out of range for the average singer. These types of songs, when executed the same way as they are on the album, will lead to poor congregational participation – unfortunately this means the worship team is just performing, and no longer leading people in worship.
To combat this problem, songs should be transposed into keys where they are easy for the congregation to sing. This is not a solution for all songs, especially those with very wide ranges – but taking the song into a more suitable key can help the situation. It is interesting to note how hymns were designed for congregational singing; their melody lines are right within the comfortable range for average singers, and also the rhythms are kept very simple and easy to follow. Food for thought!