Hi Jeff,
Sorry for the late response.
I was able to see the behavior you described above in both SP2007 and SP2010 using QWP 5.7 (both scenarios Ajax enabled/disabled). e for QWP 5.6. This confirms that is not something we introduced in the latest release but keeping the same behavior from the previous released versions.
If you need to have a validation on this "Specify your own value" field you can use Custom Actions to do that during the Save event. Please check our documentation regarding this topic using QWP help (Custom Action Help section).