It always pays in my view to use the appropriate size floating-point in whatever structures you design. Even if there is little savings in computation time with 32-bit floats, you will save memory that might be needed for other things if the precision is sufficient.
That said, it never made sense to go from double to float in a routine just to perform a few calculations. Function arguments are often promoted to double on the way in, and it makes sense to return double types for such functions.
Matrix operations and summations should be done in double precision to avoid round-off error accumulation, which is why I use doubles in rmtxop, even though it does take quite a bit of memory for large matrices.
A good example of the difference is the COLOR type, which is used throughout Radiance. There really is no need for greater precision that 32-bit float in lighting simulations, unless the color is used as an accumulator, as it is in rcontrib. There, I use a special DCOLOR type to avoid error accumulation.
Does this help?
-Greg