Interesting thread topic!
Most Japanese players do play one credit runs, without any credit feeding. This is because people are usually waiting in line to play. In the early stages of learning a game, particularly one with a high difficulty, then yes, they may credit feed if no one is around, in order to expedite learning patterns in later stages/bosses.
I have learned several games on PCB, without using any training or practice modes. Notable achievements are my 855mil ALL in MFBL JP Maniac mode with Reco, 610mil ALL in Futari 1.5 Maniac with Reco (both done before the port was released) and 114mil ALL in Ibara Kuro (long before the game was emulated). Even today I find this method of playing to be more fulfilling, with a greater sense of personal achievement, than learning a game with practice modes on a port, or save states in emulation. However, although I don't use save states at all (I'm not a fan of emulation), I have used practice modes in ports and in some cases I've scored higher this way in the long run. Particularly with chaining games like the Dodonpachis - I can't imagine learning both loops just on a PCB without any practice modes. It would take ages. Some Japanese players have done, this, however. For example, rkb49 got over 800 billion with C-Strong in DFK 1.5 before the port was released.
Basically you need to try out different strategies regularly, plan strategies when not playing the game, and study video (your own replays, or others). You also need to gain a deep understanding of the scoring system, and really study the boss patterns to see openings and exploits. This is quite game-dependent, though. Yagawa style games allow for a lot of boss milking, whereas Ikeda style games usually only have boss scoring (at the end of phases, etc) or simple boss kills.
