Is it right to presume that the top hose to heater matrix is always the hot hose?

No, that is not a good assumption. Not that I can tell you off the top of my head even which way the coolant flows through the heater matrix, it could flow either way. It can flow all the time when the HVAC isn't even putting air through the heater core, meaning both hoses would be hot all the time. I can also be that there could be a valve in the coolant stream to the heater core where, when shot off, neither side is hot. There is nothing saying it has to be engineered so the hot fluid has to go into the top and cool out the bottom. It could flow either way ... there's nothing in physics or engineering which dictates it only flows one way.