Keep ROW = NULL or DELETE it?
If the rows will be re-used frequently, it is likely better to set the values to NULL, or simply add a column is_deleted to indicate that the row has been soft-deleted.  Continuously deleting and inserting rows can lead to a heavily fragmented table, and can be a cause of performance degradation.  This is typically used for queues where there are a known number of slots (rows) in the queue, and those slots are allocated ahead of time.

If the rows will not be re-used, or will be re-used very infrequently, then it's better to deallocate the space used by those rows.