When I bought my bike, the headtube was cut very short by the LBS.

Right now the stem is almost flush with the top, maybe about 2.5MM higher than the top of the headtube.

I know that if the top of the stem is higher than the top of the headtube it can be dangerous to ride since the stem might not be secured properly.

How much room would be safe?

If my handlebars were just 5MM higher it would feel perfect for me, but I have been weary of putting a 5MM spacer on since I am not sure if it would be safe since the stem would be higher than the headtube.

Thanks