Here's my two cents (which, owing to inflation and all, isn't worth a plugged nickel). Use your naked eye for what you can. If you need more, use one of those jeweler's headband magnifiers. To get in closer, use a 3x, 4x, 10x loupe. To get down to the nitty-gritty (hairsprings, roller jewels, etc) use a 20x loupe or resort to a microscope. The more the power, the closer you have to get to your work and the more distortion you'll have around the edges. My eyeballs are older than Accutrons, so I use a 10x loupe as pretty much a standard practice.
No matter what you use, be sure to have some fun with this rewarding hobby!