minhag - Reasons for different minhagim regarding if and when to put tallit gadol over head?

I'm curious about the reasons and sources for the different customs about if and when to cover one's head with the tallit gadol during davenen. For example:

1. 
   

   cover head from after bar'chu until after silent amidah

   
   


2. 
   

   cover head from beginning of silent amidah until after kedushah of reader's repetition

   
   


3. 
   

   cover head when having an aliyah

   
   
   


4. 
   

   cover head with tallit gadol all the time

   
   


5. 
   

   never cover head with tallit gadol