I definitely think it depends on the shoes. Everyone else has posted some pretty reliable information, but at the end of the day, it is best to try on the shoes and treat them individually. I've had to go up and down within the same brands.
Rupert Sanderson: I down size 1/2 size
Jimmy Choo: generally 1/2 size down
Prada: always 1/2 size down
YSL: true to size
MB: true to size for me, except for the one time I had to 1/2 size up!
Chloe: true to size, but they don't fit my wider feet