You can think of the sun as your key light, or hair light, or (if you have some powerful lighting) even a fill light; you place the sun according to which role it has to play. Same thing about reflectors - they are just light sources, with very narrow beam and weaker than the source they are reflecting.
A midday sun is like a ceiling lamp - not really good for a key light, creating long hard downward shadows. So you either place it away (by shooting in shade) or behind the model. Since you take portraits with a tele lens, you don't have to worry too much about getting the sun in frame, so it does not have to be exactly behind the model; you do need either reflector or fill light from the front though.
Near sunset, the light quality and angle is much more suitable and can be used as any of the light sources. The light is warmer, so you if you use fill flash you should use a CTO or CTS gel (full or weaker, depending on stage of the sunset) to have the light with same color as sunlight.