It can't hurt to do a clean shutdown. It would also give the router a chance to write DHCP lease information and graph information and other things safely to flash rather than loosing the last 5 minutes to an hour worth of data.
We don't worry about it. I graph data via SNMP from my monitoring server rather than on the machine. I've not had any issues with the leases. My leases are short anyway.
This is not the definitive answer, just an opinion datapoint for you.
All my previous graph data seems to be lost.
edit: Make that all resource data . The interface data (Traffic) is still there.