Besides the obvious (from my point of view) choice between x*x and pow() I would also like to point that the gravitational constant could also be written as: 6.673 * 0.00000001 to allow those compilers without intrinsic functions to make some optimizations. It is not that hard to write a couple of important physics constants this way and the advantage may be big, since this will allow the compilers to evaluate the expression at compile-time.
