Formal or Informal?
Academic writing is about thinking, not feeling, intellect not emotion. So too much emphasis is out of place in academic assignments.
Some things which you definitely shouldn't do:
use bold or italics to emphasise a point
use bigger type to emphasise something
put in exclamations like 'Wow'
Also, you should avoid words which are informal or exaggerated ways of saying 'very'. If you want to say that something is very unusual, say it. Don't try to give extra emphasis by putting something like:
The basic principle is quite simple. An academic assignment is supposed to be the product of research and thinking. It's supposed to be thought out beforehand, and to be completely under control. If you put in lots of emphasis, it sounds as if you're surprised, or as if you're realising something for the first time, i.e. you're not in control. So don't do it.