The vocal range of the singer and the effect you want out of the instrument will dictate the tuning.
Usually, down tuning from E standard is done by cover singers trying to save a little voice on the higher notes or by bands that like the darker and lower tone of down tunings.
Try experimenting with a guitar that has slightly heavier strings and tuned down to C# or D standard. Then use a capo on the first few frets to emulate E flat, E and F standard tunings. Play and sings the songs in all of these 'standards' and see which fits your style best. Then you can commit your guitar to that tuning.