At level 0 you should be using splintered (tier I) rods. At rank 5 you should switch to pine (tier II). At rank 10 you should switch to elm (tier III) and so on.
I believe that you can use rods out of your tier range, but they won't be nearly as effective.
I'm not 100% sure of those numbers... I'll PM someone who fishes and get them to come in and clear this up

/e: yeah actually I just checked and I think that those numbers are off

I PM'd a guildmate so hopefully he can come correct me
This post was edited by NevV on Jun 25 2010 03:12pm