It is okay to bump an old topic if you have something useful to add to the discussion. I prefer having all discussions about a topic in the same place in stead of spread over lots of different threads, because it will cause people to repeat what they already said.
If you have something to say about the topic, but want to start a slightly different discussion, you should create a new topic.
For example: there's a general topic about discussing the pro's and con's of Tim Burton's movie. If you want to discuss the quality of the script, it should go in there. If you want to discuss the similarities of the characters at the engagement party with the characters in Wonderland, it should be a new topic.
However, if your reply is something like "I totally agree with that!", I'd rather have you not posting at all
--> This is okay, as it adds to the discussion
--> I'd rather not have this, because it is off-topic.
--> You shouldn't do this. It doesn't add anything to the discussion, so it isn't worth kicking an old topic for.
Jess: you can detect recent replies by looking at the icon before "Posted: [date]". They will be white if you've already read the reply, and orange if it is new. You can also use the 'view posts since last visit' link on the top right of the forum index page.