It's not the kind of music that matters in this case. It's all about how it's played.
The ability to execute a complex task "by heart" generally means that a person knows how to do something extremely well. It's a demonstration of a well practiced skill. Playing Chopin's Opus 55 No. 2 with feeling, without looking at the music, is to play the great work by heart. Operating a sophisticated camera without looking at the owner's manual is another way of playing by heart, and it's essential if a person is going to be taken seriously as a credible moviemaker. Being able to cook a favorite, delicate recipe without looking at a cookbook is certainly an expression of the same.
But life? It's easy to go through life's many motions by simply following the long-learned, regularly repeated rituals of our days as if they were learned by heart, only to discover that what might have been intimate insights were instead calcified, lifeless routines. The balance between rote recitation of one's daily lot and days played-by-heart with gusto and vibrance teeters on a swaying tightrope. In the years of practice inevitably required to become skilled at traveling our own mortal trajectory, lots of time will have inevitably racked up. We risk missing vital parts of our youth developing the complex skills of adulthood, only to find that the risks can propagate and become full fledged trends as we give up our middle and later years due to ossified routine. The moment we stop paying attention to how and why we experience the world--how we cease to intentionally pursue our days rather than simply manage our days-- is the moment we stop playing by heart and start playing by rote. That's the moment we risk playing music only according to notes on a page, without feeling, without passion, without insight.
Music is a highway to this consideration. In great performances you can hear simple melodies transformed into sublime expressions, where lesser performers might have just played what's on the scale. You can hear it in classical music and you can hear it in pop. The hook may be the definable component that makes us sit up and listen, but it's the intangible performance that always transforms schematic arpeggios into the soul-stirring sound. And, of course, this phenomenon applies to far more than music.
Creative people need to play by heart because it can free the mind to act instinctively. But creative people cannot ONLY play by heart because without revisiting the definable goals of a project, it's hard to stay fully engaged in the reason for the enterprise. Improvisation is one thing, but there's a reason directors insist on staying true to a script. Casual cooking may yield tasty kitchen creations, but sometimes it makes a difference if you actually follow your notes and favorite recipes.
Artistic lives demand that we do both. Sometimes we need our scripts, our recipes, our sheet music to keep us heading towards definable outcomes. But sometimes we need to toss our texts and ply our craft solely motivated from forces within. To do the second thing without properly developing our skills through strict practice of the first is to deceive ourselves about the nature of quality, our honest chances for genuine success. But to never abandon the security of defined instructions is to deny ourselves the potential to create something genuine and honest and new….and true.