I can't speak to the relative merits of flow vs. (what's the opposite? self-consciousness?), but I can testify that the vast majority of musicians practice dumb. They just recite, or jam; they don't target the hard part, or new ideas, although they might say they know they ought to -- and more cerebral work like ear-training or learning the combinatorics of music theory (mapping the possible chords and scales, and ways to combine them in serial or parallel) aren't even on the radar.