I have been using and recommending DD-WRT to others for a long time now, starting with v23 SP2. I've used it primarily on Buffalo hardware, such as the WHR-G54S, WHR-G54-HP, and WHR-G125.
When a new release of the firmware came out, I upgraded the firmware in the devices I have around after a waiting period, and most of them have been trouble free. (My main device, a Buffalo WHR-G54S, that began life running DD-WRT v23 SP2, has yet to ever be reset other than by a power outage.) Some, however, have not worked properly after the upgrade, and have caused reliability issues for the rest of the network to which they are attached. In these cases, hard resets and reconfiguration to the original settings fixed the problems.
I looked around the DD-WRT site and did not find anything in particular that suggested a reset to factory defaults had to be done after each firmware upgrade.
So...I'm looking to develop a consensus of answers. Is a reset to defaults required, recommended or completely unnecessary when upgrading DD-WRT firmware on a configured router that was running an earlier release?