Leading edge should always have a radius. On some models, like trainers, with large fin and stabilizer, you might get away with square L.E.

Trailing edge, however, should be square to reduce flutter on fast models (control surface oscillation).

Ideally, fin and stabilizer should not be flat but have real airfoil section (symmetrical, like NACA 0012 or something).

This does not only reduce drag, it also makes rudder and elevator more effective.

You will need a effective rudder on a 2-engine airplane, one engine may stop and you might stall a poorly designed fin if it is not very large.

Some models requires a lot of elevator to raise nose during landing flare. You will need a effective stabilizer for that.

The picture shows an old Airplane from passed times, however capable of Mach 2+. Look at the trailing edges !

/Bo