Never take your feet off the rudder.
Totally wrong.
You have control means just that - the student has control.
Don't give control to students in situations where they or you are not prepared.
To ride any of the controls while the student thinks that they are the only one influencing the controls causes all sorts of problems.
If it is critical eg simulated engine failure in a twin then
a) don't do it at such a low height that will cause serious problems should they get it wrong the first few times they try it i.e. push the wrong pedal; and
b) always Place your foot (note the singular) in such a position so as to not be on the pedal but to prevent it moving in the wrong direction. This also put's your foot in the ideal position to take over with minimal delay should no rudder be applied.
That is the great thing about the rudder pedals, if the student must apply right rudder and must not apply left rudder, putting your foot just in front of the right pedal both prevents left rudder application and gives minimal delay should the student not apply the rudder at all.
Same thing can be applied to spin entry and kicking off the drift angle in a crosswind landing.
So never ever ride the controls - any of them!!