Can you let me now which abattoir is offering this, they'll need a complete slapmark kit and a couple of minutes to set yours up. Pigs are slapped with the abattoir's number as part of meat hygiene inspections, was this possibly what they meant?
The wrong pig back is very rare, and yes there are examples of it, just as there are examples of people walking out of their house to be immediately hit by a lorry, but you still step outside. Slapmarking does give better confirmation (esp if the abattoir is going to butcher as well), but this is a choice.
On stress, slapping just before slaughter should not add stress unless very badly done, most pigs just react for seconds to the slap.
Technically it would be illegal under PRIMO 2011 to move the pig to the abattoir wihtout either a slapmark or a tag.
Final thought is that if several people left their pigs at the abattoir for slapping this would increase the risk of your pig getting the wrong number, and you getting the wrong pig back.