data storage for timestamp data type?

The timestamp data type in postgreSQL has the option for a parameter 'p' that defines the precision from 0-6. The documentation indicates 8 bytes as the default size for the column.

If I use timestamp(3) as an example, will postgres still allocate the entire 8 bytes for the column or reduce it?

Thanks!
Chirag

PS. By the way, I think clustering question hit the list 3 times by accident last week. I apologize, I used the wrong e-mail address and assumed the first 2 didn't make it.