@TheWhiteAmbit : I normally wouldn't hesitate to use bool to enable/disable parts of code like this. Since this is not a branch that changes so often from true to false, CPU branch prediction will take good care of it and at best it would be a minor micro-optimation . 
Also since you will probably will call small stuff like that with literal values eg.  RotationLookAt(x,x,true,out x) if the function gets inline ,compilers/JIT will probably remove the if entirely