In what types of loops is it best to use the #pragma unroll directive in CUDA?

In CUDA it is possible to unroll loops using the #pragma unroll directive to improve performance by increasing instruction level parallelism. The #pragma can optionally be followed by a number that specifies how many times the loop must be unrolled.

Unfortunately the docs do not give specific directions on when this directive should be used. Since small loops with a known trip count are already unrolled by the compiler, should #pragma unroll be used on larger loops? On small loops with a variable counter? And what about the optional number of unrolls? Also is there recommended documentation about cuda specific loop unrolling?