Although if your input is 12V or more, I would also wonder about whether the 8V you measured is the actual maximum value, or if sometimes (depending on temperature, input voltage etc.) it may go higher still. I think a well-designed converter really should ramp up from 0 to the setpoint on startup, and not generate an excessive voltage under any conditions.
I like the sub-$1 DC-DC converters on ebay for 12V, 24V -> 5V based on the MP1584EN chip. Now you have me wondering if they have an overshoot on startup- I've never actually looked at that.