It does depend on the type of valve and actuator.  If you tell me what kind I can get specific.  Balls and butterflies are not generally a problem, except sometimes the actuators, which can be a problem on any type of valve.  Gate and globe valves will tend to have crud built up in guides and in the bonnet that may prevent full opening and may compromise operation (increase operating effort), backseats don't work if there is something on them.  Foot valves, swing checks, stop checks and tilting disc checks just won't work upside down, since they are gravity operated/assisted. Nozzle, Silent and dual plate checks are fine upside down, unless upside down means flow down.  Some solenoid valves are position sensitive, and almost all work slightly better with the plungers vertical.  

If the gland should leak the liquid will come down on the actuator.
If you have a positioner,  it could be difficult to service.