Personally I don't think it's ever appropriate. Anyone interested in Astrology can figure out the sign. Additionally there are different types of Astrology. Some people follow Tropical astrology and some follow Sidereal astrology. The general consensus is that it's trivial. The only possible time is when properly sourced the individual in question states that the believe in astrology. See archived discussions below:
Wikipedia:Village pump (proposals)/Archive 39#Astrological signs on articles
Wikipedia:Categories_for_deletion/Log/2006_November_7#Category:People_by_Astrological_Sign
Wikipedia:Categories_for_deletion/Log/2006_December_25#Category:Lists_of_notable_persons_by_astrological_sign
P.S. Don't forget to sign your talk posts.