Newly built cities should always prefer plains. Currently there is no focus which would ensure that.
Cities which are near its maximum population and/or happines should switch to MaxProduction focus (exception- if city build settlers then it is nearly always better to use HurryProduction).
Variable Exploitatoin Focuses poses many problems for Terraforming Managers. If city changes its focus frequently then it is difficult to predict exploited locations. For example newly built city might have Growth Focus but quickly switch to hurry production. Terraformer should not build too many improvements using Growth focus.
As long as AI uses Despotism there is no such problem. Plains are far better than any other terraint type so it is always used first.