I highly recommend supplemental lighting. Its broadens the light spectrum and plants love it. That being said dont spend too much on extra lighting.. you just want to broad the spectrum the cheapest way possible... if youre growing with low MH then supplement with high nanometers. If youre growing with high hps then supplement your low nanometers.
I personally do just this.
I have some led light bulbs all same brand.
1 pair of 2700k 17 watt 1600 lumen ea.
1 pair of 5000k 15 watt 1600 lumens each.
When I'm using the 2700k to supplement my mh it adds %11.03 to the total light output. And adds %7.90 to the electricity consumed for lighting.
When using the 5000k to supplement my hps it adds %5.81 to the total light output. And adds %6.97 to the total electricity consumed for lighting.
I do believe plants benefit from more full light spectrums.
So adding a few supplemental led bulbs is a good idea. They are cheap to operate and they produce little heat.