That's the myth that has propagated since the series ended, but I recall JMS's Usenet postings when the show was in production, and back then he was always quite candid about how radically he was willing to change his plans in response to changing circumstances. He had certain key points he wanted to hit, but it wasn't as precise as having every episode plotted out in advance. For one thing, he made major changes from the pilot to the series, dumping three of the originally intended regulars (Takashima, Kyle, and Lyta). For another, he didn't write the whole first or second season. At the start, the show was like any other in that it had multiple staff and freelance writers.
After all, JMS is the man whose personal motto as a writer is "No plan has ever survived an encounter with the enemy." The reason I know that saying is because I read it in The Complete Book of Scriptwriting, which JMS wrote. This is a man who knows the importance of keeping his plans loose and flexible -- who knows that trying to work out precise episode-by-episode details in advance is a waste of energy, because things are always going to change or be discovered along the way.
So the level of detail in his advance plans for B5 has been hugely exaggerated in fan mythology. Yes, he had the 5-year arc planned out, but not in scene-by-scene detail or anything like that.
