I am now in the habit of turning off the camera anytime I mount or remove a lens, flash or memory card. I think the latter is unnecessary because the camera turns itself off when the memory card door is opened, but I do it anyway… because it’s a habit.

I haven’t always done this and the worst I’ve ever experienced from leaving the camera on through any of these changes is a Speedlite inadvertently firing (if it is also on). Likewise, I have never has a USB flash drive go bad if I’ve removed it without unbooting it properly from my computer. But I know people who have fried their flash drives by doing so. Better safe than sorry with any kind of electronics like this.