If it's leaking around the lock nut, it's actually leaking between the rubber seal on the bottom of the fill valve and the bottom of the tank. If you have to pull that tank again, I would definitely try sanding the area. Depending upon what a PITA it is to remove and reinstall that tank, you also might try swapping out the fill valve while the tank is off. You can use a Korky 528MP (528MPK on the Lowe's web site) fill valve, which has a silver cap. You don't need to buy the green one from Toto, which is more expensive as they do a smaller, hand-made run of them for Toto.
If the toilet is brand-new, I would speak to Toto about sending you a new fill valve if a leak developed in the rubber around the old one. If it's new, it is also possible your installer just didn't give that nut that holds the valve in place an extra snug before hooking it up to the water supply. We don't often see leaks developing around the fill valve gasket, but it can happen, particularly if anyone is using a bleach-based cleanser inside the tank.
So, I would see if snugging (and sanding before snugging) it does the trick. If not, I would swap the fill valve, not because there is anything wrong with the valve per se, but because that gasket may not be holding.