That is a sucky schematic...It isn't at all clear which position on the switch is 'on', or whether the 12V is the supply voltage, or an output voltage. Can you give us more background on this, ie where the circuit came from, and what it's used for? It'd probably help lots.

For starters though, it is generally a good idea to tie that 'not used' trim pot connection to the middle (ground) connection. The theory behind that generalisation is that when trim pots fail, they usually fail because the middle connection fails. As the circuit stands when that happens the trimpot in effect becomes an open circuit, with the left leg of the regulator floating, ie unpredictable. With the 'not used' connection tied to the middle, when the pot fails it will fail to it's maximum resistance. That still may not be a good thing, but tends to be a more graceful degradation. Again, more background would help to clarify that.