There are some witness parameters like a minimum account creation fee, a maximum block size, an annual percentage rate for SBD and a price feed.
Sometimes we talk it over, sometimes we don't. We don't need an agreement here. In the end it is at sole discretion of a witness. Of course we don't need a fork as those parameters can be altered by witnesses by design. Effective value for the network is computed using median from values of witnesses that generated blocks.
What we can't alter is anything that is a part of the consensus, for example, we can't change the algorithm being used in computing curation rewards that we agreed on in the latest hard fork.