I've never heard that before, there were clashes between Alyson and Sarah by all accounts, but I find it hard to believe that Joss would let Sarah dictate how much dialogue he gave the supporting cast

ETA Joss is supposedly a real stickler for actors not changing dialogue in any way and he wants it said exactly as written, that's why he got so annoyed with Donald Sunderland on the Buffy movie because he kept rewriting everything. Joss complained in interviews that DS was an actor, not a writer, and all his little rewrites didn't make any sense and were ruining the movie. That's part of why he was happy to move to TV, because working on moves led him to believe that the writer got no respect when it come to keeping their words intact