In my opinion:
- creation topics can always be necroposted, since there is no expirationdate on a good creation
- project topics can be necroposted by their original poster or anyone wanting to pick it up again if it is not finished yet, for finished projects... just... no
- suggestion topics can be necroposted if there is any relevant new info, or if it is a really good suggestion that has not been looked into yet
- support topics can be necroposted if your problem has not been dealt with yet after a long amount of time. They are I think usually locked when the problem is solved tho.
- application topics can be necroposted, but there's no much use in it...
- helpful articles topics can be necroposted if there is a problem that's not properly explained or something's not up to date
- offtopic... depends on the topic ^_^
- introductions, not much use, but why not