Is the word whiskers more like UK English and "sideburns" more like US English? I see the term originates from "Ambrose Burnside" who was American so the word "whisker(s)" can be older than the word "sideburn".
I haven't heard anyone in the UK use the word whiskers to mean sideburns or any facial hair. The word sideburns is used and means this http://en.wikipedia.org/wiki/Sideburns
Good examples of this are the hair on the faces of Elvis Presley and Bradley Wiggins.
In the US, the use of the term whiskers overall has declined overall since the 1930s. The referenced chart does not distinguish between whiskers on animals and humans. However, most of the decline is likely to relate to the human kind, since the term is still the standard one to refer to long muzzle hair on cats, dogs, etc.
In describing human facial hair, whiskers is used to refer to hair on the whole face, but it is not very common and is more likely to be used in a largely humorous or joking manner. It would generally be considered old-fashioned, although most dictionaries do not list it as archaic.
Sideburns has a more consistant modern usage, but is only used to refer to the whiskers descending in front of the ears, usually in a fairly narrow band (the broader, longer version often being called muttonchops)