For tarot spreads that require a “Significator” card to represent the querent or the topic of interest, the challenge has always been how to go about selecting the “right” card. The old books advised using a querent’s appearance (light-haired, dark-haired, fair-skinned, swarthy, etc.), gender and relative age to make the call. This provided a rough approximation that was typically matched to one of the sixteen court cards when the intent was to designate a person and not an issue. A more recent twist on this has been to use the querent’s astrological “birth-sign” and the court card assigned to it in the Chaldean system of correspondences.
The fallacy inherent in this approach is that the astrological “signature” of an individual’s personality isn’t limited to the zodiacal placement of the Sun at birth. The basic formula for this has always been Sun, Moon and Ascending signs as the three primary building-blocks of the persona as it is viewed by the world at large. Since it isn’t reasonable to pinpoint a single card for this triplicity, using only the Sun-sign amounts to another generalization that may be even less accurate than the old way. Aleister Crowley recommended applying what you know about the querent to make your choice, but in the case of walk-in clients, that is essentially zero unless you resort to “cold reading” clues. I usually make a preliminary judgment based on biological gender and apparent age, and then ask the client to look over the handful of cards that meet the criteria and choose one for the purpose. In rare cases I may ask for their birth date, but it could be perceived as an intrusion on their privacy. Since I sometimes use the astrological associations for the cards as a back-up resource for interpretation, I might ask how much a sitter knows about astrology in order to tailor my approach, and the Sun-sign may be revealed during that discussion.
For a topic rather than a person, the choice can be simpler. For a work-related reading I would use one of the minor Pentacles that suggest labor: the 3 of Pentacles for collaborative work and the 8 of Pentacles for self-employment. For a budding romance, the 2 of Cups is a straightforward option. It is possible to assign the astrological correspondences from the Chaldean system for this purpose, but the subject usually speaks for itself and going that far is seldom necessary. The major (trump) cards all have elemental, zodiacal or planetary associations that can be correlated to the topic as well, although I don’t often use trump cards to represent people or situations.
In practice, the Significator card as an “avatar” for the sitter in the spread is more a part of what I call the “theater of tarot” than a vital component of the reading; it arguably makes the person feel more engaged in the process but adds little or nothing to the narrative. I have stopped using them in most cases, so the opportunities to bring in astrological assumptions are few.