I think it's better to start a new one, unless the old one very specifically is almost exactly like the one you are starting.
The reasons why we don't like bumping old threads, and no offense to those who do it:
A lot of old threads drift down to the bottom and are eventually pruned. Sometimes people use the search engine, they find threads they want to reply to, but these threads may be older than newer threads that are more appropriate to post in.
Other times, the old threads are actually going against some rules, specifically new rules we put into effect, so due to this, we will close old threads that are bumped as they are brought to our attention.
And yet other times, threads that would otherwise belong in a different sub-forum will be posted in Other Drugs (for example), it will drift to the bottom, and only until someone bumps it back to the top will we notice this.
In this case, sometimes we may close it, send it to BDD, or just send it to the OD Archive.
If the thread is still worthy of remaining open, we may just let it stay open and see if it generates any more discussion. Other times, we may feel that it has ran its course and has served its purpose.
The only way posting in an old thread would constitute as breaking rules (as far as I can see at the moment) is if you bump a year old + thread to say "+1" or "I agree." - this does nothing to generate more conversation worthy of keeping the thread open.
If you are bumping an old thread, make sure you are also not echoing someone else's post. If you have the same thing to say that someone else already said, it is likely not worthwhile to bump it up to the first page again.