Thank you for the response, I will read through the man pages and post what I find for others. I had previously posed this question on the pnp4nagios site and they responded saying rrdtool was not built for infrequent checks like this.
EDIT (21 Jul 2011 11:41 GMT-5):
The file with the default heartbeat and step size for pnp4nagios is set here: .../pnp4nagios/etc/process_perfdata.cfg
My understanding is that the heartbeat will set the maximum time to wait before an UNKNOWN value is returned and step will set the minimum interval between accepting a new value so I have left step alone and have changed the default heartbeat to be 1week (604800). I will repost later once I am able to see if it works. From what I understand, the only downside for a large heartbeat is that the graph will "hold" a value for a long time if no new data is received rather than set it to UNKNOWN.