I do a lot of wine tasting evenings run by a local MW. Spitting is most definitely only an option, not a requirement or 'supposed to'. If you are driving then yes, you are better to spit or only take tiny sip or two of each one. Uusually more than one sip of each is required as good to be able to compare one glass to another... when else can you have 4 or more wines lined up beside each other? In a big tasting you may also want to spit if you want to try dozens of different wines and actually be able to still talk.
Not spitting on a night like this will not get you legless, but certainly merry and above the legal limit. For a one evening event such as this I would certainly suggest trying to find a taxi/public transport option and thoroughly enjoy your evening.
Whichever you do, have fun and enjoy.