There are rumors around about pxUpdateDateTime but they are false.
The truth is, if you obj-open-by-handle with locking and successfully get the lock, and you write a value in your own property X, then obj-save and commit, when you read back the record later again with locking, if no one else has touched property X, you should see exactly the value you wrote there. It doesn't matter what you do to pxupdateDateTime.
However, a bug was fixed recently for Pega 7.1.7. The symptom was that if one requestor does obj-open-by-handle on a record with locking just as another requestor has just unlocked it, there is the danger that the one doing the new locking will get stale data on their page.
If you have evidence that this is happening to you, you may put in a ticket requesting HFix-24300 . /Eric