It's not that they killed every tenth member of whatever village they were adding to the roman empire that day, but rather than they conscripted some of them as soldiers and then killed every tenth of those who didn't obey.
I think the currency point stands though: value the money our soldiers carry, tax collectors will be along for it shortly, welcome to the Roman Empire.
Well OK but a decimation was no small thing, it was documented to have happened just a few times in Roman recorded history.
I'd hesitate to think they employed such measures just to push the ur-Euro, especially since it doesn't seem like it would be a very effective incentive compared to the utility of the currency itself.
"I fished for a few weeks and got these coins that last forever and are easy to carry and now I can buy a horse at the horse market!"
So the currency decimation thing smells like a myth/exaggeration to me, but I admit it's just a hunch.