Email etiquette questions?

Is it okay to write casually to a professional about professional matters, like if you want to set a date to shadow them (obviously my case), if you've spoken with them casually about it before? Or should the stiff rules of email etiquette still be maintained? I don't mean casually as in typing things like 'omg' or 'lol' or not capitalizing or punctuating correctly, I just mean in a not-so-business-like voice.