Flushing is a mental hangover from hydro and chemical growing. If your grow medium is correctly prepared to sustain plants throughout their lifes cycles without chemical supplements you've created a living soil complete with microbe colonies that service plant requirements. What would be the purpose of flushing?
Taste is dependent on plant genetics, nothing else. Think about it; if plant taste/smoke was dependent on what's in the grow medium or (if one believes wild marketing claims) supplements, most would taste like bat/bird guano, decomposed cow manure and whatever else is in the grow medium.
From an issue of Cannabis Culture, words of Ed Rosenthal:
"The genetic code determines an organism's
potential to grow.
The environment determines how that potential is acted upon. Think of the genetics code as a set of plans and alternative plans that direct the plant's life progresses. It holds the information that the plant needs to grow. For instance, if the plant is shaded it senses the lack of light and produces hormones(gibberellins) that induce the stem to grow taller so it can reach the light.
The potential of a plant to produce THC and terpines is affected tremendously by the environment. Total production of cannabinoids is mostly dependent on growth because a large plant produces more THC then a small one, even if they are of equal potency. Thus, growing a large healthy plant utilizing intense light and plentiful fertilizers increases total THC production.
Additionally, plants grown under intense light produce a higher percentage of THC than those grown under dimmer light. High intensity light increases both growth and potency. In high-quality plants, the percentage of THC also increases as the amount of UV-B light increases.
Nutrients also affect the quality of the high. In an experiment on fertilizers done in the year 2000, clones produced vastly different quantities of bud depending on the fertilizers used.
The buds looked different too, and each produced subtly different odors."
The keyword here is
gene expression. It is why even genetically identical twins don't end up looking
exactly alike, nor will they necessarily have the same health problems growing up; because diet, level of physical activity, other personal preferences/habits and a plethora of other environmental factors (exposure to pollutants, toxins, drugs, biological agents, etc.) will influence their development. Certain diseases (obesity, diabetes, cancers) for instance, a person might have a genetic predisposition to them but the outcome is ultimately reliant on confounding factors.
As far as cannabis goes, characteristics like a particular taste or smell are most notable when the plant was grown in ideal conditions and thus allowed to express its genetic potential to the fullest. Odor and taste are the result of the hydrocarbon profile, which includes those terpenes and phenolic compounds. While the genetics of a plant will primarily direct the manner in which these compounds are to be synthesized, the environment (which includes fertilizers used) influences when and how much- at least to some degree. Of course, a plant couldn't express any traits beyond that of its genetic potential, but the environmental conditions determine if such traits will be expressed to the fullest.
I've also heard that sulfur is an important component of the odoriferous components of cannabis, and thus is necessary in abundance in order for a plant to express odor to its full potential.
Obviously we can't taste the fertilizers themselves in the buds we smoke, because the fertilizers themselves are not taken up by the plant- the nutritional components
of those organic fertilizers are. The plant doesn't directly absorb the guanos, manures or meals; they are broken down by microbes into the more basic components (nitrogen, phosphorous, potassium, calcium, magnesium, sulfur, etc.) which
are available to the roots, or to the mycelium that assist the roots in the assimilation. These components are then utilized by the plant; assembled into amino acids, chloroplasts and other plant structures, or stored.
Beyond all of that, the herb needs to be dried slowly and properly cured; there is still a lot going on inside of the cells that make up the buds we just harvested. The THC undergoes a change into its more psychoactive forms, and the chlorophyll breaks down. So while I have never smoked weed that tasted like guano, I have smoked some that tasted like chemicals because it was likely grown with synthetics and not properly flushed.
The onliest change in late season watering that any reg. gardner does is "turning off the water", [or tapering it off] to speed up the harvest, and gitter' done before hard frost. 
