There are no specific rules regarding greeting cards. Space is always an issue that needs to be considered. Some volunteer groups have guidelines on how they transcribe greeting cards, but to my knowledge, there are no specific rules. Some issues to consider are as follows:
Determine margin -- width of greeting card will determine this
Transcribe the greeting on braille paper first, and proofread (this will tell you how much space you need for the greeting)
Indentation -- probably best 1/1 unless verse is a poem, then use 1/3.
Don't try to follow the same lines as print copy, as they will not coincide with braille
Omit unneccessary capitalization -- sometimes entire phrases are capitalized in print --this is unnecessary in braille.
Remember: These are not rules ... just considerations.
Hope this helps!