Good idea, after trying it a number of times, it has some downsides. Most calendar applications cannot clearly display 5 minutes past, and the meeting appears to start on the hour visually. One of the attendees ends up dialling in at the hour, and then everyone gets a notification that the meeting has started.

Half of the people who get the notification click "join" without checking. This ends up with a half-populated meeting room. The issue becomes obvious, and somebody says, "Let's dial back in 5 mins", and drops off. Half of the people like the idea and drop off, while the rest decide to stay and chat.

Meanwhile, some of those who dropped off see this as a great opportunity to grab a brew. That inadvertently triggers some water-cooler, kettle-corner chats, and they end up running late for the 5-past. The rest usually get engaged in something else to make use of 5 minutes, and miss 5-past since no new notifications are issued due to the people already chatting in the meeting :)