if that is the only switch to turn on the electronics then you should turn it off....if you have a key (and like Jag says) that is only working as a kill switch...then it shouldn't matter....too be safe, I would just turn it off...
edit....after reading your other thread about the bike not turning on....I'm betting that this runs some of the electrics and that is what killed your battery....also I'm pretty sure (not knowing Harley's I can't say this is 100%, but I am 90% sure) that is NOT an OEM switch....so turn it off and find out why that one is there instead of a factory switch
Could it be a fan motor switch? I don't know what bike you have. I had test ridden a bike when I was looking for a bike that the owner had rigged such a switch for the fan motor as the fan sensor had gone out on it and he didn't want to pay the $300 to replace it. I didn't buy the bike.....
I read this in another thread, you probably did too.
"Another possibility is that you have a parasitic draw on your bike which is bleeding down the battery. That doesn't normally happen very often on Harleys, unless someone installed an aftermarket accessory that's wired directly off the battery or fuse block, and doesn't get power through the ignition switch. "
You use that to tell the motorcycle ECM (or carburetor) if you are ON or OFF the bike. It was used on bikes before weight sensors on the seats --- like what sets off the seat belt warning chime on cars.
The million dollar question is if you are OFF the bike, say, putzing around, but the bike is idling, should the switch be on or off? YOu can find threads on the Internet that go on FOREVER on the merits of each approach.
I ascribe to the "if your ass isn't on the seat, keep it OFF" line of thinking.