it is not a recommendation to change the letter cases when turning them into QR code, it is an option that exists to server very special cases like if you wanted to print the address on a very small surface like a physical bitcoin otherwise it is best to stick to lower case because the other wallet that reads it may not be as sophisticated to recognize upper case!

For presentation, lowercase is usually preferable, but inside QR codes uppercase SHOULD be used, as those permit the use of alphanumeric mode, which is 45% more compact than the normal byte mode.