We insist that all single-engine approaches are flown fully stabilised on hitting the glide/ final descent point.
This means (A320) flaps 3 or FULL (we dont specify) at G/S* thus preventing a big buildup of thrust just when you dont want it - rudder trim etc out of kilter!
You'll hopefully never have to do one for real so practice what's easiest or what the SOP says if it's mandatory.
Where it's optional it depends on what your normal 2 eng ILS procedure is. The advantage of flying it as per a two engine ILS is that the pitch attitude (picture ) is the same - only the thrust required is different. Advantage of F3 is less thrust reqd (about -5%) and in easier G/A. Just remember that if doing F3 app then the G/A is immediately to F2.