FLAC/OGG tags are case insensitive and are stored in the file with only upper case alphabetic characters. ID3 and MPEG-4 tags are case sensitive. The FLAC/OGG tags are only converted to the necessary internal format when they are written to the file. If you are using Yate fields or unassigned items that Yate directly processes (eg. Discogs Release Id), you do not have to worry about the alphabetic case. This is because the application normalizes the internal names of items it directly knows about.
If however, you are defining your own metadata this can be problematic. If you create a UDTI named My Metadata, the UDTI will appear as MY METADATA after being saved and reloaded. The User Defined action function can be set to be case insensitive and that is one method of getting around the issue.
You have another option. Names which are mapped to UDTIs are treated as normalization values and affect the display of the metadata being loaded. If you do not map My Metadata it will be implicitly be mapped to a UDTI but the display name will not be preserved. However, if you map a UDTI to My Mapping, Yate will handle the modification of the name when loading and writing the value.