That's the only way to set a default. Unfortunately, that only applies to the 'Rating' - and not the frame.
One option is to change your panel schedule templates to look at the circuit rating as opposed to the Frame Rating.
Another option, if you need to quickly change all frame ratings to lets say a 20A circuit:
Create a new Schedule, a Electrical Circuit Schedule.
Add the Frame and/or Rating Fields.
Sort by Frame and/or Rating, and uncheck 'Itemize Every instance'
Click ok and view your schedule.
You should now be able to change the value in the schedule, which will apply to all like values.
If you have some circuits that need to be different, you'll have to manipulate the schedule sorting (use Panel & Circuit parameters to your advantage here) and 'Itemize every Instance' checkbox to get the output best suited for the task.
Hope that helps!