We know JK Rowling gave Harry Potter her own birth date for (presumably) purely sentimental reasons. She has, I think, been remarkably detailed on the dates of birth for a huge number of her characters. Significance is lent to their names, their wand properties and their Hogwarts houses, and so the birthdays don't seem to be discussed often. Is there any evidence that suggests that Rowling used star signs or any other such ideas to lend meaning when she detailed the dates of birth for so very many of her characters?
Certainly many characters have meaningful birthdays, chosen through a variety of methods.
For example, J.K. Rowling chose for Fred and George to have been born on April 1, due to their penchant for pranks and humor:
Fred and George were born - when else? - on April Fool's Day.
I am not aware of Rowling (to date) admitting to having considered astrological signs when creating her characters. However, I suspect she may have, at least in a few cases. Attempts to correlate characters' personalities with their signs are beset with the same flaws that hinder real-life astrology, but some other associations are a bit too pat:
Harry's sign is Leo, the Lion. The lion, of course, is the symbol of Gryffindor house. Since Harry shares Rowling's birthday, this must have come later, but I still wonder whether it is a coincidence. Perhaps Rowling chose the house symbol for this reason. In particular, we know five Leos (Harry, Percy Weasley, Ginny Weasley, Celestina Warbeck, and Neville Longbottom) and all are Gryffindors.
Luna Lovegood, basically a hippy, was born under Aquarius.