This depends a lot on what kind of soft corals you are trying to have. If you have gonipora / sun coral / daisy / Christmas tree / feather duster, etc, you do need to feed them. If all you have is frogspawm, hammer, rics etc, then it is not necessary.
Do pay close attention to your water quality after the feeding. SPS is very demanding when it comes to nitrate level. You don't want to loss your SPS b/c of feeding too much to your LPS
I used to do spot feeding but that becomes diff b/c the minute the coral sense chg in current, all the polys retract. Nowdays I flooded the tank with spectral vital, coral frenzy for 2 hrs every other day, then run the skimmer to take out the rest of the food. That seems to work pretty well for me.