My Ipod was over a year old before I rockboxed it. Although I hadn't really used it much in that time as I didn't like the sound quality of MP3s with the original firmware.
Since I've rockboxed it, I've listened to it a lot more and so far the HD is working OK. I don't know if RB uses the HD more than the original firmware. I do notice it spinning up more at startup but I doubt that will make a difference.
I'd recommend using your next Ipod with the original firmware for a while just to make sure it isn't a hardware problem first.
IMO It is worth Rockboxing Ipods as they are so much better than with the original firmware.
MP3s sound much better and the ability to play FLAC is great, but the best thing of all time, is being able to get rid of ITUNES.