Unlikely that the non-return valves will stick unless the system has water in it and they will rust or the springs rust and break. The Control Valve can stick when it is worn if you do park the tractor with a load on the 3 point and with the Engine stopped. When the Engine is running the Control Valve is rocked from side to side in an effort to prevent sticking. While there shouldn't be a load on the control valve because of the non-return valves (8 of them) holding the pressure, if these get worn, as they do, there will be some pressure leaking and that will hold the Control Valve. Why park with the 3 point in the raised position anyway, it can be dangerous should someone move the Control Levers (children?)and the implement will crash down.The hydraulics were never designed to hold up indefinitely as there is no need for this. The leak-down you get when you do park it with the implement raised is from various parts of the system and it is just wear and tear, but when the Pump is running it will be more than capable of taking care of small leaks like that. Usual places to leak are the Lift Cylinder and the non-return valves and the Control Valve in the Pump. There is nothing to prevent you parking the tractor with an implement attached in the raised position, but if the Control Valve sticks, it doesn't always free-off very easily, so the easy way is to lower the 3 point when you park the tractor. I have attached some pictures of the Control Valve and the Non-return valves(12-17)..the vertical "lines" on the control valve are washer or spacers that separate the different pressures in the Pump, these do wear with the continual sliding action of the Control Valve, so the holes get bigger and allow pressure to leak. These valves are a precision fit but do wear over time and dirty oil definitely does not help.
It does say that you should always lower the 3 point when parking in some versions of the operators manuals. Why don't you like the idea of lowering the 3 point each time, it would be interesting to read your view on this....John(UK)....firstname.lastname@example.org