You can't really hurt the engine by running on Reserve the whole time. But you are going to need to make sure you check your fuel level frequently. The nice thing about On running out is you always still have Reserve. But if you are run Reserve the whole time, when you run out of gas, you've REALLY run out of gas.
The valve is pretty low tech - open to ON and gas flows (unless it is clogged like yours). Open to Reserve and gas flows (unless it too is clogged).
Given that there is gunk in the valve at all is an indication that the rest of the fuel system is probably getting gummed up as well - including the carbs. So fuel line replacement, valve cleaning, and carb rebuilds are in your near future - see my links below to get started on the project.