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I don't think it really matters. That said, I have mine coming out to the right side away from the battery. With the bike leaning to the left the right side is open and easier to access.
Use a little strip of velcro on the frame cross member behind the motor and a strip around the battery tender connector and it stays out of the way when not in use. Use some industrial strength velcro and a tiny plastic tie around the connector velcro and it will never fall off.
As long as it isn't touching the exhaust it doesnt really matter. Put it to whichever side is closest to the wall that you park it by. Mine came with one and it was on the left side so I moved it to the right side
I have mine on the left side mainly because I changed the plugs to 1/4" audio plugs and use the same direct connection to the battery for my heated gloves and cloths. The female plug is mounted to a bracket on my adjustable backrest lever.