For best performance and comfort, yes. Any variable speed equipment, mini split or conventional ducted split gives you a lot of wiggle room in sizing, but calculations should be done anyway.

Yes, absolutely. Always size your HVAC equipment. If you oversize the system you will cool the space quickly, but it won't pull out the humidity effectively, leaving you with a cool, clammy room. If you undersize the system will run and run (and you won't have a humidity problem) but you won't get the temperature you're aiming for either.

That said, as ionized_gw mentioned, you have some pretty good wiggle room with ductless systems because of their variable speed technology. You can be "close enough" on the sizing, just like with a game of horseshoes! :)

There is what is probably an under appreciated advantage to oversizing variable speed heat pump equipment. The systems run at a higher efficiency at part load. I am finding this true for boilers as well as heat pumps. I expect it is true for furnaces too. Any of this equipment probably "saturates" the heat exchangers as they approach top operating capacity. In fact, HPMS equipment has a sometimes quotes overcapacity that is seen for a few minutes after start-up. Inefficiency could also come from increased resistance to refrigerant flow at higher rates. I am no engineer so my reasoning is all hand-waving. I wonder what happens in multi units when all indoor heads are operating vs. only one in this respect. If I knew, I might operate my system differently sometimes.