This is a question that is often asked, and yet is almost impossible for anyone else to tell you. As can be seen above, polishing and restoring a Japanese sword is a major and costly undertaking. Spending $3000 restoring a sword that might be only worth $2500 after restoration doesn't make financial sense. On the other hand, for a good sword, the ability to be able to study it, learn from it, and appreciate it afterwards is reward enough for many people.
The first thing to decide is whether you have a decent sword or not. This cannot usually be seen merely from online photos or if it is badly out of polish. Fatal flaws can be hidden under rust, and the blade could be tired (worn down from repeated polishes)
The best thing to do is get it to a professional togishi who can open a window in the blade and see what the hada and hamon look like. This involves polishing a small part of the blade, and will allow the polisher to tell you if the steel is good and if the hamon looks good. With his advice, you can usually decide whether to proceeed or not.
The other way is to take it to a sword show or meeting near you. Although there isn't always access to these everywhere, in many countries there are sword appreciation groups and shows. Questions posted on a sword forum will usually tell you if there is a group, meeting or advanced collector near you.
It is usually not financially viable to have a sword polished merely to sell it. Adding the cost of the polish to the cost of the sword makes the end result fairly expensive. Unless you turn up a sword by a good smith for little money, you might ultimately lose money on the deal. In this case, it is best to sell the sword out of polish and let the future owner decide how to proceeed.
If the sword has been identified by experts to be by a big name smith or a very good piece, then it is recommended to have it restored, as the end result will be a sword that shows the smith's work as it was intended, and the blade should increase it's value accordingly. But this involves positive identification of the sword and its maker, preferably through a shinsa.
In some cases though, collectors will spend the money on a polish so that they can enjoy the piece and learn from it. There is no doubt that there is no better way to study Nihonto than to look at good swords, in polish. Little can be learned from a rusted and stained lump of steel. In some cases it is sufficient to just care for the sword and enjoy it for what it is, and leave the restoration up to future generations.
In the end...the question of "should I have it polished" comes down to your own finances and how dedicated you are to preserving the blade. Cheap polishes by amateur togishi remove metal and shorten the life of the sword. They should be avoided at all costs.
Self-study will assist you in judging the shape and style of a blade, and perhaps allow you to see something under the rust and grime that indicates the blade has potential. Feel free to ask for opinions on the internet forums, but take the replies as a guide only, and try and have the blade evaluated in hand by someone qualified.
Accept the fact that during the polishing process, fatal flaws or blisters or open grain can become visible that were otherwise invisible. This is one of the risks that come with this hobby. On the other hand, there is nothing as satisfying as seeing a blade that has come back from the polisher in all its glory, showing wonderful grain and a temperline that shows everything as the original smith intended. This is what collecting is all about.
So in conclusion, there is no fast and easy answer to the question of whether you should have a sword polished or not. If money is no object, then yes. If you are like the rest of us, you will want to take your time, get other opinions and weigh the financial outlay vs what you will gain afterwards before you decide.