How safe is the single engine taxi out for twin engine aircraft?
I am not aware of mechanics watching civil aircraft start up. They usually have far better things to do. Aircrew are responsible for the aircraft at this point.

The question on a twin is about directional control not safety. How easy is it to taxi on one engine without losing control or stressing the aircraft or tyres in any way? I think this probably depends on the aircraft, some will be easier than others. Also if its a piston twin, you don't want to wait for the other engine to warm up when you are ready for departure. Some four engined aircraft might taxi on two engines - one either side but I've never heard of twins doing this routinely.