2 out of 3 for me.
Currently that means matching bars and stem (Ritchey WCS 260 and evocurve), and Canyon seatpost. Both black/white so don’t look too jaring together. It’s not like a stem can ‘fit’ better than another stem, they all come in 10mm increments. so why not pick it on asthetic grounds to match the bars (which are a personal choice) or seatpost, which was probably picked on asthetic/weight/price grounds and the stem will probably fit the same criteria.
It’s not so much trying to match stuff, just a case of why not match it? It won’t affect the ride positively or negatively more than the sum of the individual components but the perception is that if the rider has bothered to match components, has colour matched cable ends, neat cable routing crossed under the downtube, neatly wound bar tape and a saddle that looks significanly more worn than the rest of the bike etc, then they’ve probably spent time setting it up right, and it probbly rides very nicely indeed and the owner is a ‘propper cyclist’ rather than just someone on a bike.
Whereas a cobbled together winter hack says exactly the same thing as the nice summer bike, but only in mid winter.