Most people don't have their speedo readjusted because the difference in total wheel/tire diameter and speedometer reading often ends up being pretty negligible. The height of the sidewall is the most important part, you can get very close to stock size depending on what sidewall size you pick. In that case your speedo will be very nearly correct. My speedo is slightly slow and I just take that into account every time I look at it... if it shows 70, I'm actually going about 72. This usually doesn't mean the difference between getting a speeding ticket or not. One thing it also affects though, is your odometer mileage! My car has more miles on it than my odometer shows, and after many miles, I bet those fractions of miles add up.
But the way you adjust your speedo is with a simple adjuster pin. It doesn't cost much and I don't believe it's very difficult.