Sighting onto prism from TotalStation 

Quick question- when sighting onto the target prism it is usually over long distances for me, I sight using the yellow guide triangles at the side of the prism. But when the prism is close you can see the white lines in the centre of the prism (the edges of the prism mirrors) which form a star at the centre of the prism. When sighting on I notice this centre point does not match the guide triangles, particularly when sighting down or up-hill. 

Can someone briefly explain how this will affect the traverse, and also tell me which is best to sight onto- the white lines within the prism or using the guide triangles?!

Thank you for any help.