if the dictator messes with the currency everyone uses as you have suggested, he will wreck the economy and have very hungry and angry peasants on his doorstep.
believe it or not, dictators have to appeal to and appease the masses more than a democratically-elected government, because the gov't can say 'fk off, you voted us in, and you can vote us out again in 5 years, but until then, tough titty.'

we have seen with the lack of an uprising over brexit that the british people have given up being angry with their 'democratically-elected' rulers.

anyway, if the dictator of the future can simply switch off (and steal) his political opponents' and dissenters' e-wallets, it will have no effect on the broad economy but in fact give him the ability to re-distribute this stolen wealth to his supporters to encourage loyalty. it will give him immense power - even more power than the central and commercial and investment bank owners have now, which is saying something.

i hope i'm getting my point across - of course you can have dictators without a cashless society, and you can have a cashless society without a dictator. but if you get a dictator who can take control of a cashless society, well then you have the ultimate tyranny, and no one who remains in that society can escape his whims.