Damper close to main supply vs damper close to the register end: what is the difference between the two positions?
The biggest difference I see is noise.  The damper at the register likes to whistle when I have it mostly closed.

Also I personally want to install dampers on my main trunks so I can control air flow to each level, this is where dampers on the lines excel at.  Controlling air flow to areas of your house.  In the summer I want more airflow upstairs and in the winter I want more flow downstairs.  If your system is trunked/branched right, you can do this with only a couple of dampers.