No, they're not. All the scripts for seasons 1-6 were already written in the first 10 minutes when they sat down and conceived the show. In fact, they hired all the actors, and filmed every scene of every episode in the first three weeks.
OF COURSE they are making it up as they go along -- that's what writers do, duh! They live in the real world and have to deal with networks and actors' schedules and fan feedback and a million other things, and they have allowed for some flexibility in their storytelling (thank God!). But that doesn't mean that they don't have an overall plan. They do, and they have been true to it from the very start, and it is the best show ever. Come on people, these guys are amazing storytellers. Anyone who thinks differently is, well, you know...
stoooopid.