I just feel that where you can, especially if you are writing songs for an album, or to perform as a live set, they should ideally be consistent. To have one song sung all Proclaimers style, followed by a bon jovi-esque american number may just seem silly. Whats wrong with deciding what your identity is, then, where possible, sticking to it? I try where I can to sing my songs in as 'english' a way as I can, hopefully without sounding naff. I am guilty of waivering from time to time, if the song seems to need a bit of a mainstream sound.
I've pretty much accepted that my style of music, at my standard and ability, will never be something that the masses will buy so I just try to stay true to my roots. There are millions of artists the world over who sing with an american accent, my aim is to be myself.