Is it OK to switch to PHP 7.2 now that Drupal 7.6.1 supports it?
The latest versions of CiviCRM run fine with PHP 7.2.  However, a) this has only been true a few months, b) Drupal 7 has only very recently become 7.2-compatible, c) most major Linux distributions don't ship with 7.2 by default.  I'd say those three factors account for few sites running it.

Note that as of CiviCRM 5.8.0, known PHP 7.2 import issues have been fixed.  CiviCRM 5.9.0 will handle the other known PHP 7.2 issue, which only occurs on certain types of reports.