All sound is subjective. The algorithm that was written by Sonos was designed to do as much as possible, I think, to mitigate poor speaker locations and provide as “flat” or “normal”, whatever those really mean, a sound to the user.
You should always tailor any sound system to your desires. I don’t think I have any, including my Sonos, that are set to “flat” as they come from the factory. I’ve tweaked each one of them for the environment (like my convertible) and the speakers. It’s likely most people don’t need or want to adjust after Trueplay, but there’s nothing wrong whatsoever if you do. Trueplay isn’t “perfection”, it’s as close as they can get to a programmed “ideal” as defined by something other than your ears.
It’s your system. Modify the sound properties to your ears, your musical style.