APE tags are case sensitive. However, in order to complicate the issue, it is forbidden to use APE tags which only differ in case. So, for all intent and purpose, the names are essentially case insensitive. An attempt is made to be case preserving. When tags are written to the file, tag names which only differ by alphabetic case are coalesced. 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.
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 desired display name may not be preserved. However, if you map a UDTI to My Mapping, Yate will handle the modification of all tag names with different alphabetic case representations when loading and writing the value.