You can get away with shooting an undersize bullet with black powder due to the high impact at ignition, the bullet is shortened and fills the barrel. With smokeless the bullet has to be slightly oversize to the grooves and throat dimensions, the lower impact at ignition and burn do not fit the bullet to the riflings as in the case of black powder. 

There are plenty of differences between the two powders and it is better to pick one powder and study the bullets that work in that type of powder then spend the same amount of effort with the other powder. Heck, to confuse things even more there are those that mix the two powders and further confuse the issue. There are some bullet designs that are somewhat universal but for the best in accuracy one can do better with specific bullet designs associated to the individual powder type.

Then there is the consideration that no matter what the mold says, depending on your alloy and your casting temp, the bullet that comes out may be different. If you are using smokeless, you definitely don't want it too small. If it is a few thousandths over you can always size it down which with some molds (Lyman?) may give you a more uniform bullet anyway. A mold from a custom maker who asks you about the alloy you will be using and the minimum size you need can generally be trusted. Many if not most mass produced molds will generally err on the plus size.