Is it ever okay to ignore a warning about an inconsistent Database schema?
It could be okay, for a time, until the module that has an out of date schema attempts to read, save, update, or delete data from an out-of-date database table. At that point, you'd get inconsistent data in your database at least, and possibly end up with a corrupted database or have your site return the WSOD (white screen of death) at worst.

The warning simply means there is at least one update that hasn't yet been run. It's always a good idea to run updates by visiting update.php or using drush updatedb whenever you update any modules or Drupal core.