As a background designer, I enjoy drawing cities and researching different styles of architecture to give them realistic details while adding my own flavor to them. Although, certainly, the rules of perspective apply to such drawings, I often eschew my ruler during the clean up (or inking phase, I'll post a series of drawings that will illustrate this process in the future).
As the years have gone by, I've become more and more reluctant to rule out even my roughs. It's not that I shun the rules and think that my lines are perfect without the ruler, but since your pen or pencil runs along an edge that is constant, there is a sense in which the ruler kills the character of your line. Oddly, even though your hand can run along a straight edge far more confidently and quickly than without it, I find that, mentally, things go faster when I draw freehand.
Still, some lines just call for the ruler. On occasion, the freehand line can get a little too wobbly, especially with any dose of caffeine, so there's also the possibility of having to bring in the ruler to straighten things up. Some artists even hold their breath during some of the longer and more crucial lines. For me, this practice was, in part, inspired by an exhibit I'd seen at the satirical Museum of Jurassic Technology. It's down the street from where I used to live and my former apartment manager's husband was it's curator. The exhibit highlighted a man whose microscopic, fully posed and painted dust sculptures required him to stroke his one hair brush in between heartbeats.
Now, before I start off some young artists on the wrong foot, I should add that I did first learn correct perspective with rulers and strict rules on creating the illusion of a realistic depth. Repeated use of these techniques then allowed me to tackle more complex perspectives with angled and curved planes, multiple vanishing points & a bunch of other jargon filled aspects. While I can't say I've fully mastered perspective, I can see that I have improved in my understanding of it. The understanding has, in turn, provided the solid foundation that enables me to produce convincing perspectives in freehand. Others may discover their own way of coming to this point, but that has been my process thus far.
Child prodigies and budding Mozarts aside, most musicians must learn how to generate notes on an instrument along with scales and chords and some way of remembering them in sequences (most of the time by learning the language of notes on a staff with the various markings), as they tackle the basic harmonies and melodies of simple songs. With practice, reading those notes and commanding the body to respond accordingly become second nature allowing the focus of a performance to shift from those basic concerns to more complex ones that might add to or accent certain points of a piece. Even the timing and understanding of those things can grow, until that musician is performing the most difficult of concertos in front of a captivated audience. Of course, not every musician strives for such mastery nor can everyone attain it, but I am confident that those who do have suffered long hours of practice mixed in with various triumphs & failures, that refined their skills and demanded a piece of themselves to pour into its artistry.
I hope to draw "concertos" some day.