What should be the fill-factor for a table where only fixed-width columns will be updated? We have a table where we are planning to update timestamp fields only.

In the first import these columns will be nulls then we will update them as the computation finished on these rows.

The question is; should we set a fill-factor lower than 100 for this table or PostgreSQL will allocate a fixed size for these columns so lowering the fill-factor makes no sense?
Postgres doesn't store nulls directly in the field but rather keeps a bitmap of null values. 
So using 100 will be problematic. 

What you can do is use some default value such as 1/1/1970 just to allocate the spot for the field.