So I have been saying it right this whole time.
Yes you have, because there's not exactly a wrong way. Unlike Technics, the subject of another detailed and ultimately inconclusive discussion on pronunciation, it's a Japanese-derived word. But in both cases, they contain an "i" sound, which in Japanese falls somewhere between the short "i" in a word like "bit" (and it's worth mentioning that few languages, even European ones, have an exact match for this English sound), and the double e in "keep".
The "ni" of Nikko and Technics is short (in Japanese), but Japanese also uses long vowel sounds (there are five basic vowel sounds, and five lengthened versions of the same sounds), and the distinction is important as otherwise there would be many more homophones than there already are. In phonetically written Japanese, this lengthening is always clearly shown, not with diacritics, but with a symbol in katakana, and simply with an additional hiragana when the word is written in hiragana.
When Japanese is written in romaji (Roman script), long vowels can be correctly indicated in various ways. In practice, it's often dropped in names, so Tokyo with two long vowels is spelled as it would be if it had none. The city of Niigata is one example where the lengthened "i" sound is seen even in the English spelling, and can be said to be "accurate". Nikko on the other hand is not: the short "i" and double "k" correctly indicate pronunciation, but the "o" does not tell you that it should be pronounced in Japanese with a long "o" (actually, macrons are sometimes used, but personally I wouldn't bother, because if you're going to use them, it commits you to always attempting to make the distinction - good luck with Sapporo, Hokkaido, Fukuoka, Hiroshima, Oyama, Kochi, Kofu, Kobe, Hyogo, Sado, Nagano, Tohoku, etc. You see the problem!
In speech, incorrect lengthening or shortening (usually by non-Japanese, obviously) can genuinely cause confusion - asking directions for taking trains is fertile ground for this - in the same way, perhaps, as you might be if a non-native speaker seemed to say floor when they meant flower.
But back to Nikko, the reason I originally said there isn't really a wrong way to pronounce some of these Japan brand names is that in English they're all anglicized anyway, and anglicization itself isn't standardized. There are variations between one English speaking country and another, and also within individual countries. As well as that, it doesn't necessarily settle the question to dig up a commercial in Japanese and follow the pronunciation there. With Technics, there wasn't even agreement on how many syllables could be heard (five for me, three for many people), and with the vowel sounds, that "i" sound in particular, I think people were in effect matching the Japanese to their own pronunciation (neeks or nix) rather than hearing it exactly as pronounced, which I would say is neither. And that's before we even consider which syllables to stress, where there are a multitude of differences between American and British English, for example, and where Japanese often applies equal stress to all the syllables in a word.