Drupal 7.22 has just been released.
This is a bug-fixing release, so upgrading immediately is recommended but not absolutely essential. Over 100 bugs are fixed in Drupal 7.22.
However, this is a good opportunity to check up on all your sites.
Drupal 7 releases have been coming rapidly over the last few months. Drupal 7 had no releases in its first 5 months, but has now had 7 releases in the last 7 months. 4 of those releases contained security fixes.
We’ve moved rapidly from 7.16 in October to 7.22 in April, so please make sure that all your sites are up-to-date.