I thought that these were always considered slang, and that during World War II they became slurs (partly because of their regular use in war propaganda). I don't imagine that there's a time when either term would have been considered a "proper" way to refer to Japanese people, although I do imagine that before World War II they probably wouldn't have been considered insults and might have been considered affectionate.

If my guess about that is right, then you could compare it (pre-WW2 only!) to "Aussie" and "Kiwi" (for Australians and New Zealanders), which nobody would say is "the" name for these groups, but which is also not an insult. Although this is complicated because these are, I think, also autonyms for these fellow English-speakers. Maybe another example is "frogs" for the French, which is mildly disparaging but usually not a horrible insult, at least in contexts that are otherwise respects.