Adding fingering numbers to scores

What you have to do is:
1. go to the first note you want to finger, in the right or left hand,
2. Press alt+c to go to the Create menu,
3. Press x for text,
4. Press o for other text,
5. press f till jaws says "fingering", which is f twice
6. and finally, press Alt + one of the number keys in the number row between 1 and 
5. After that, space leeds you to the next note and skeeps the rests, of 
course. So, you can do it quickly.
To exit the fingering text field, I think you can press tab or escape, but 
I'm not sure what works better.
If the player is A beginner, I think you can add fingering to each note, but 
if not, then it's better to finger just what you expect to be difficult, as 
you said, or whatever needs, in your point of view, A special fingering 
which will help the player play it better.
An other thing that I'd like to add, is that when you finger A scale or A 
part of A scale, you should finger at least the specific notes that should 
be played with the first finger...
Briefly, I think that the best guideline is to ask yourself what helps the 
player and doesn't confuse his/her eyes with too much information.