There is noting wrong with direct editing in the file list, i.e., delayed double click to enter editing mode. It's perfectly fine to do it this way 
What I meant was that the way you've set up the column for the custom field via View → Columns... determines the way the field name is written to the file. If you use %MyField% as Field in the configuration, it would use MyField when writing the custom tag to the file. If you use %MYFIELD%, MYFIELD is written.
This is purely cosmetic and doesn't make a difference from a technical perspective. If you want consistency, I'd use all uppercase field names — because the extended tag dialog and the Tag Panel also use those.