Smokie, I fell off my seat at that. Fortunately I had finished taxying
otherwise there may have been an incident.
On a serious note, our company policy is to allow the F/O as much taxy practice as possible, except onto stand. After all, how else do you gain this experience ? radius of turn versus speed, nosewheel position and aircraft body length etc. The Captain will have his feet on rudder pedals and assume control immediately if required but this really is an area where F/O's should be allowed to gain confidence, especially prior to command assessment.
XL ONE, no need to worry. If it was an aircraft with only 1 tiller (757 has 2) and Captain was incapacitated, the F/O would bring the aircraft to a halt on the runway. If necessary, a tug could be attached to convey you to your stand.