Firstly, we always produce one version of the arrangement in the key of the original recording. When completed we then consider producing additional versions in different keys. If we determine that the original key makes playing it difficult we may move the key up or down slightly to produce a ‘friendlier’ version for the musicians to play. For example, if a piece was originally written in the key of E major concert (4 sharps) this would produce trumpet parts in the key of F# (6 sharps) and Eb Alto sax parts would have their parts in 7 sharps.
By lowering the piece by just a semitone (to Eb concert), this would put the trumpet and alto player in F and C respectively. We don't simply consider just a player’s ability to play in 'awkward' keys, one of our biggest considerations is the vocal range. If, for example, the singer in the original key is singing extremely high it would be pointless to transpose the arrangement even higher. Generally, our decisions for putting the arrangement into a specific key is made using a combination of experience and common sense.
Also refer to How do I check if I am able to sing in the key of your arrangement?