I'd have hoped that any speedo would retain programmed settings irrespective of battery status. Quite a few cars lose any user generated setting in the ignition map, and resort to default settings in the case of battery removal, but the needed relearning process is generally not too onerous, and will be back to pretty much normal after a short drive.
For a modern five speeder, something like this best:
which when wired correctly, when operated will both stop the engine and not blow up the alternator.
The following type is ok if it is never operated on a moving engine, and is what I've fitted in a hidden location to the two speeder. But if there is any chance of operation on a running engine, you need the former.
Both pictures from Demon Tweaks, and more at:
Demon Tweaks battery isolators 
