I've seen both methods employed by people on here. I'm curious why people chose to do what they did and if they had any regrets or significant frustrations with what they chose. For the sake of explanation I somewhat define the methods as follows:
Master Planning - Layout has a very detailed plan for all facets, not just the track routing. For example, wiring runs for things like building and accessories are done all at once before the train is ever running. The method could be seen as Design, Tweak
Design, Think Some More, Finalize Design, Build, Build, Build, Build, Done & Run.
Organic Building - Could have a track plan or might be completely freeform. Emphasis is getting the trains running. Then go back and add in things like scenery and accessories. Described as Plan, Lay Track, Run, Add Some Stuff, Tweak & Move Things, Run Some More, Add More, Eventually Be Finished.
Personally I'm of the Organic School. That's mostly because I want to have something running. I also fear I'd never get started if I went with a master planned route. Some frustrations with the organic method are that I have to live with situations that are less than ideal - wiring around existing stuff, stuff in less than perfect spots, etc. I still consider myelf as having many years in the hobby left ahead of me, so I half expect to eventually have a fully master planned layout - perhaps when I have more space and time and years of experience in the bank.