It depends on what substrate/method your using.
In supersoil there may not be the be the need to feed at all and just giving them plain water throughout the grow.
In hydro/coco then feed every time unless they need a flush, if watering coco drain to waste at 30% run off, then you shouldn't need to flush coco during the grow, only using plain water as an end of grow flush.
Even in flush a very weak nute solution can be more effective than just plain water as it gives the nutrient ions in the soil something to bind to.
Is there a substrate/medium you are looking for specific answers for?