This is a question without a definite answer, because I doubt anyone can reliably draw a line between periods of "personal style is not important" and "personal style is important". It seems most likely to have been influenced by the fairly high number of people involved in anti-fashion groups in the late 19th and 20th centuries (which does fall under "people ... subscribing to the same ideas wearing similar clothes"), such as the dress reformers, aesthetes, beatniks, hippies, and punks - the more subcultures exist, the greater the mainstream social pressure to mimic their philosophies of dressing to express oneself - but even before the late 20th century, the concept existed.

Historically, one can find references to "personal style" that are clearly relating to poise, bearing, etc. - aspects of one's person that do not relate to the choice of clothing (and so aren't displays of taste, which relates to one's aesthetic sensibility in choosing what to wear and buy) but affect how one is perceived by others. This has nothing to do with individualism in dress, however. Both this form of personal style and taste were dominant before individualism, leading to an overall conformity to current styles (with modifications where a general fashion was very unbecoming to an individual).

At the same time, it must be said that individual expression was certainly possible before individualism was a huge force in fashion. This is most evident when it comes to royalty, especially in the early modern period: monarchs like Henry VIII and Louis XIV are well known for using their clothing to project an image of personal power and authority. Okay, that's not really what you were thinking of, but there are more germane examples. Elizabeth I is known for portraits in which she appears in clothing that is a version of fashionable dress, customized beyond the addition of more gold lace, a bigger hoop, etc. etc. One instance is the Rainbow Portrait, where Elizabeth wears a gown with a huge serpent embroidered on the arm - not a standard ornament on late 16th century dress - and a cloak embroidered with eyes and ears. To me, it seems likely that the cloak's painting was imagined by the artist at the sitter's request, but the serpent looks like it's been done from reality; it's considered to be a symbol of wisdom, representing Elizabeth's own qualities. Another highly individualized portrait is the one at Hardwick Hall, in which Elizabeth wears a petticoat and stomacher painted or embroidered with dozens of plants and animals - again, above and beyond just an ornate version of fashionable dress, this probably is more symbolism intended to express Elizabeth's personal qualities. To go in another direction, Marie Antoinette wore and had herself painted in a riding habit with breeches and no petticoats, very similarly to a portrait of Louis XIV - this sartorial choice is considered to have been an expression of her desire for more personal power.