In order to see all of the activity in the sword's hamon and hada (temper and grain), the sword has to be in polish. Unlike most collectibles, it isn't wrong to polish a Japanese sword and restore it properly. In this respect, they are more like fine old oil paintings which get cleaned and repaired, than they are like fine old furniture on which the original surface is valued. Age patina on a Japanese sword is only valued on the tang (nakago) and this is the part that is never cleaned.
This is also where a good togishi will correct the lines, shape and features of the blade, and bring it close to what the original smith intended. Polishing a Japanese sword involves an incredibly complex routine which doesn't open up the surface grain, but allows the texture to be seen, and at the same time brings out all of the incredible activities in the steel. This is way to ultimately appreciate a Nihonto, and can only be seen properly when polishing has been done.
However, every time a sword is polished some of the sword is lost to the polishing stones. Eventually the skin steel will be gone, the less refined and coarser core steel will show through, and the sword's artistic, historic, and monetary value will suffer. For this reason, only a properly trained polisher should ever polish a sword. Proper training takes years of study, almost always in Japan; no one can learn to polish Japanese swords by reading books and watching videos. Amateur polishers remove too much of the skin and leave the sword with an improper polish, which will need to be redone properly, which will remove even more of the sword.
Japanese swords don't have to be polished to survive; there is nothing about the polish that protects the blade from corrosion. If you own a sword in reasonably OK old polish, there is nothing wrong in enjoying it as it is and preserving it. These is still a lot that can be enjoyed. Seek the correct advice before doing anything, and try and see a few swords in full polish so that you will know what is expected and what to look for.