If you have a knowledgeable, reputable dealer, that is where you should go for information. Or, if you know a good repair shop you can also get answers there. I don't think sewing machines need regular tuning up. They need to be taken care of with regular cleaning and maintenance as shown in your manual. When you use your machine all the time, you become tuned-in to its performance and if something seems amiss, take it in for checking. Otherwise, save your money.
I'm basing this on the experience of using the same machine for nearly 30 years without servicing. I'm also thinking of tools in my husband's shop. His machinery is just as intricate as my machine and it operates in a dusty environment without any more than regular maintenance for years.
Maybe you have a machine that needs tune-ups, and your dealer will be able to tell you that.