Taxied out single engine on 4 of my last 10 or so sectors (taxied in single engine on 9 of them - the only exception being a very short run into the gate) - it's like anything - the more familiar you get, the less of a drama it is. It takes a little bit of judgement, but then again what doesn't? Obviously there is a fine balance between saving a drop of fuel and being caught out, so as a rule of thumb if there are 3 aircraft at the holding point or less I'd be inclined to be good to go on arrival, I stick to familiar airfields and make sure the guy/girl next to me is 100% happy to go along with it, but within the bounds of reason, why not?