Top Up the Charge??

Let's say I tell the car to be charged up at 7:00 AM. At 8 AM, it's all done, and showing 100%, but also showing the time to charge is 40 minutes. IOW, it's actually slightly less that 100% charged.

I can plug it in, turn off the charging timer, and top up the charge to get a few extra miles.

Would that be a good idea or a bad idea?

Bad idea. In fact it could void your car's battery warranty. There's verbage in the manual about how repeatedly topping off if the charge level is already near-full is considered abuse and will void that warranty.

I recommend at least having one charge bar disappear before plugging back in again. That ensures the car's battery has gone down to at least 85% if not lower.

The 40 minutes remaining could be cell-balancing. It's recommended that you do this at least occasionally even if you normally don't require a full charge. The car is truly not done until all the charging lights have shut off.

Not worth it IMO but it's doable and Nissan even posted about it...

Nissan Discusses Method For Topping Off LEAF Battery (0.7%)

The last 30-40 minute is cell balancing indeed (verified by my EVSEs reports). It's a good idea to do that occasionally if not always.

Do not top off. Even the gas stations tell you not to.

Good to know. Glad I asked.