If you have OmniFocus for Mac, the default due time is set in the Preferences, Data tab. It will sync to your iPhone, etc.
If you don't have OmniFocus for Mac, it's a little trickier to adjust the default due time. In Safari on your iPhone (or iPod Touch, or iPad), you need to visit the URL
Code:
omnifocus:///change-setting?DefaultDueTime=22:00
Yes, it really has three '/' characters in a row, that's not a mistake. You'll want to change "22:00" to the appropriate 24-hour clock value for your needs.