Necroposting is not allowed.If the topic is trending with no posts in it since a week or two and you truly have something constructive to contribute to it,you may.If the last post made in the topic was a month or months back ,more like a dead thread,you aren't suppose to resurrect it because once you do,the sig spamming follows.The rules are subjective to change in special cases ,depending on how relevant your post is.
If you are reported for necroposting ,your posts will be deleted.
Where did you get this from? According to
this post by mprep on the rules list, so long as it does not break any of the other rules necroposting is completely allowed.
As said previously in the thread, so long as your post abides by the rules then it is allowed.