I've heard that at least some libraries find that it is faster and more efficient to have a partial table of sin's and cos's, then use the addition identities to fill in any one that they want:

sin(x + y) = sin(x)cos(y) + sin(y)cos(x)
cos(x + y) = cos(x)cos(y) - sin(x)sin(y)

Cheers,
Ben