In Leonard Nimoy's autobiography I am Spock, near the end of Chapter 2, Nimoy claims that he had the idea for Spock alone to have the pointy sideburns, but that they later gave them to everyone.
An interesting sidebar: I had suggested pointed sideburns as a specifically Vulcan look for Spock. Apparently Vulcans are intergalactic trendsetters, because you'll notice everybody in the Federation now sports those pointed sideburns.
This does suggest that pointed sideburns were Nimoy's idea, but this could still be in-line with Gustav Mendoza's claim that his father Richard Mendoza came up with the style for Spock, and Nimoy approved and suggested it be just for Vulcans. Or possibly they came up with the idea together.
Gustav does say that Richard came up with it for Spock, not the entire crew who later sported it, so that part is now from two sources.