Whether you should use FULLSCAN or the default percentage depends on your data and the inserts/updates/deletes happening on the table. You could perhaps try with the default and if you have bad query plans, go for FULLSCAN.
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DBCC UPDATEUSAGE corrects the rows, used, reserved, and dpages columns of the sysindexes table for tables and clustered indexes. It's different from update statistics. And you can turn off 'auto update statistics' if you like maintenance plan to handle it.