Do you have to feather pedal if you have a enginer conversion kit?

AFAIK, the way the Enginer kit works, it does NOT change the behavior of the Prius in terms of how easily the ICE kicks in nor can it get around the max EV speed of ~41 mph.

If you add a PHEV kit, you'll probably want to add an EV button (2nd gens in the US never got it).

 

You can add one with a $2 switch from Radio Shack.

The only thing the Enginer kit does is (almost) continuously add charge to the OEM battery. When the OEM is at a higher SOC, it uses the electric motor more.

With a coolant temp spoofer, I don't even hardly need the EV button, it just depends on how hard you want to accelerate.... The only time I use the EV button is if I know I want to accelerate more quickly on electric power alone (i.e., override the prius computer).

To answer the OP, feathering the pedal still helps encourage the Prius computer to turn off the ICE when you are in S3/S4.