You’re welcome, Planetypus.
That’s the general “rule”, but I guess I should also mention that there are a few exceptions.
For, example, you can use too in a negative sentence if it is not at the end of the sentence:
“He, too, didn’t go.” = He didn’t go either.
Another exception is when you use too in a sentence that is asking for confirmation of an affirmative. For example, if I was at a big party and I believe you were also there and I now want to ask you to confirm my belief, I might ask you:
“Weren’t you there, too?” = “I was there and I think you were there, too. Am I right?”
For the above sentence, if you use either, the meaning changes (because you would then be confirming a negative):
“Weren’t you there either?” = “I was not there and I think you were not there either. Am I right?”