Switch on either of the +/- leads, Ken. I repurposed my old roof sign switch to further improve the environment. Another option is to switch off the ignition for a few seconds after every job (AFAIK all SDA installations have the meter switched through the ignition) but that'd be a right pain in the bottom, IMO.
Mine's a Hale Microtax 5 (as is Ken's unless he's changed it recently) Horse and the switch works fine. I only almost ran out of paper* because I relocated my printer inside a little mini glove box and forgot to switch it off after an extremely rare request for a paper receipt - out of sight, out of mind.
* have to catch mine before the roll expires 'cos the feeder buttons don't work anymore so, bar taking the thing apart, the new roll needs to be fed behind the old roll and a receipt reprint carried out. Fortunately it generally only needs changing when it goes yellow, after c.2.8 years.