I don't think anyone has ever done purely a from light source tracing, because it doesn't make sense. Tracing from camera is not an approximation. Brute-force

I don't think anyone has ever done purely a from light source tracing, because it doesn't make sense. Tracing from camera is not an approximation. Brute-force path tracing is random sampling from camera to every direction and bouncing/interacting until light source is hit. This is the standard Monte Carlo path tracing method which is unbiased and arrives at the accurate solution given enough time. This is unbiased, accurate and very simple to implement, but very slow to compute. Metropolis and photon mapping are bidirectional methods where rays are selectively traced back from light source to forward direction, to speed up the path tracing computation.