Indeed it should be, maybe it's the 5V supply that's not stable. I've increased the BOD level on a couple projects to 4.3V because I want the µC to go into reset ASAP when the power starts failing. Works fine near as I can tell. I have noticed that some of these inexpensive plug supplies (wall warts, AC/DC adapters) do some interesting things when they're unplugged. I have some where the voltage falls, then there is a quick blip where it increases again. This is one of the reasons I tried the 4.3V BOD level.