Is it OK to remove field values or revisions by deleting individual lines from the field table in the database?
It is better to Use the Node Revision Delete module to delete and manage your revisions.


  The Node Revision Delete module allows you to manage the revisions of the Node according to your choice. It helps you to keep the specific number of revisions for the node. This module provides you the flexibility for applying the revision delete for the specific content type and run it on the specific time. You can manage your settings from the Node Revision Delete Administration Page.