Yes.
It's a bit like drink driving. You can feel completely normal with a BG of 4, but remember a lot of people think they are perfectly ok to drive with a two and a half pints inside them. Hypos (and alcohol) affect your decision making.
Only unlike the alcohol-filled brain, the hypo situation can get worse while you actually are driving. Because you are busy concentrating, you don't always pick up the signs right away. Also, if you drive .e.g. on a motorway, it isn't always easy to stop when you first notice the signs, making it worse still.
When I was diagnosed a year ago, 4 was the lower limit, not 5. They apparently put it up because people would slip lower without being aware of it.
The people who make these rules are experts who have the statistics in front of them.
Some people in their position would say, 'nah we'll just stop all diabetics driving, it's too dangerous.'
But they haven't said that, they've said: 'We'll give these people a chance to be responsible.'
I REALLY appreciate that because I wouldn't have my job if i couldn't drive.
On the other hand, if someone killed my kid, or my husband, or whatever because they'd been in a rush and judged it wrong..... I would be sooo ANGRY.
So in short, yes it's important to eat before you drive.