This is an old question, and I will try to give the old best answer.
The point at which water is connected to the heating system is known as the neutral point.
This should be on the return to the boiler, and be the nearest connection before the return from the system enters the boiler.
Some systems have open vents, which are run to above a small (feed and expansion tank), typically in an attic in a domestic, or even a commercial building.
Such vents should be the first connection off the flow pipe from the boiler.
The best place for the pump (s) is just after this vent.
Many modern systems are pressurised and do not have open vents.
The best place to connect the water feed is still on the return at the boiler.
The best place for the pump(s) is still on the flow side.
This keeps the bulk of the system under positive pressure from a combination of the
pressure imposed by the system water connection, and then when the pump is running, the pressure provided by the pump.
The only area where the pump pressure in the system is negative is within the boiler.
The pressure imposed by the water connection should be great enough, and positive, to ensure that even in the boiler the combined pressure will be positive.
The full result is that you do not get boiling due to negative pressure at the pump suction (entry point), and do not get air drawn into the system through the almost microscopically thin gaps around valves, and other connections in the system.