When we speak of a substance smelling or tasting, this is not felt to be “agentive”—that is, we do not think of the substance as doing something, producing the smell, but as having the smell. This is why, for instance, we do not say that a perfume smells ‘well’ but that it smells ‘good’—an adjective attributed to the subject.
The -er suffix is agentive—it ordinarily designates someone who does something—so a smeller or taster will ordinarily be understood to mean someone who perceives smell or taste, not something which has a smell, unless there is something explicit in the context which suggests otherwise. You may use these words without fear of being misunderstood.
Note, however, that someone who smells as an occupation (in a perfumery, for instance, or for a food or wine merchant) will probably have a job title such as “smell tester” or “odor tester”—and will sometimes be referred to as “the nose” by colleagues. Similarly, beer-taster Roy Desrochers is a trained “sensory consultant” and is the “sensory practice leader” for his company.