I think many learn that way first. For me, its whatever key fit my vocal range often, depending on song. Higher end in F/G, lower was in C. Then I had major throat surgery almost 6 years ago and a wider airspace lowered my range to Bb. When I had a female lead singer in the band, I would play to her range, sometimes transposing when she sang the same songs I did when solo, sometimes playing in her keys. Instrumentals I often play in original keys; many old traditional songs don't have a set key.