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You can see chapter 3 of "Expert Oracle Exadata" for all details on hcc and pctfree.
HCC should use a pctfree of 0 as compression units (CUs) are only written once - rows can not be added to a CU at a later point in time so why leave free space that can never be used?
HCC tables definitely default to 0 PCT_FREE. As Greg said, updates do not add data to compression units, rather they cause the updated row to be migrated to another (non-HCC) block. So usually there is no point in leaving extra space in the blocks. You can however create a table with a non-default PCT_FREE. The setting will be ignored for rows inserted via direct path though (i.e. HCC compressed rows). Non-direct path inserts (which will not use HCC but revert to OLTP compression) will use the non-default PCT_FREE setting though.