I have a dash cam but I just plug it in after the engine has started and unplug after the ignition is turned off. Reason is, upon starting it can be a little unstable i.e turn key its on, turn again to start its off, when engine has started it switches back on again.
Just out of interest, which dash cam did you go for? I want to install one when I pick my car up, and have looked at a few...there seem to be so many, and would be good to know how yours is performing!
I bought a Snooper DVR 3HD, it also does speed cameras, very pleased with it, the others I have are a Snooper DVR 2HD and a roadhawk in our other vehicles ( to lazy to keep moving one to another ) all equally as good.