For the Dash-8, I'd start them both up when making a right turn off of the gate. For a single engine taxi, after pushback there may not be enough space in front of the plane to get some forward momentum going before making a 90 degree right turn away from the dead engine, or enough space behind the plane to make a 270 degree left turn "the long way around" into the dead engine.

So I'd start them both up, use differential power, a free-castoring nosewheel, and a tap on the inside brake to turn off the gate. Once away from the gate and moving straight ahead, the hydraulic nosewheel steering comes on, and the left engine gets feathered for a normal taxi time in order to save some gas and reduce the noise. If the taxi time is going to be excessive, then #1 will be shut down after having at least 30 seconds in feather.

For the Dash 8-300, I would start them both up every single time, but #1 would usually stay in feather until getting close to takeoff. For the -300, it's real easy to exceed the maximum steering angle for the hydraulic nosewheel steering when trying to make tight turns during single engine taxi. I've seen too many guys exceed the steering limit, the hydraulic steering kicks off, and now there is no way to get the nosewheel straightened out unless you stop to take the time to now start the #1 engine and then use differential power to bring the nosewheel back into the steering limits.

So on the -300, I'd start them both up and leave #1 in feather. Whenever there was a time that I might exceed the maximum steering angle, I'd then just have to lift the #1 condition level out of feather and up to the idle detent in order to use differential power to straighten out the nosewheel at the completion of the turn. Once back on a relatively straight stretch of taxiway, I would put the #1 engine back into feather. Very rarely, almost never would I ever truly "single engine" taxi the -300. The #1 engine was always in feather.