Well, contested calls may favour one or the other team. But I would argue that it isn’t biased towards any side. You can easily make up a similar scenario where the offense is favored.
“Stalling 6, 7, 8” – “Fast count” – “Contest”. If the marker counted in one second intervals but the thrower still insists on the fast count the offense suddenly gets two seconds for free.
So I think maximum 6 is a good compromise.