Hieber, the only reason to change springs is because of sag, or if you're not happy with the stiffness of the spring. I've had to change springs on one car because the car sagged about one inch from specification. The car was handling somewhat squirrely, trying to wander to each side, which is another sign of sagging springs. You'll need to find the specification for your car and measurement location (usually a frame point). I imagine a lot of people needlessly change springs when it's really a weak shock or loose suspension point causing problems.
A tip for Ron and others... I've made a table of the standard size wrenches and sockets I use with the converted metric size listed next to it and taped it to my toolbox. This way, whenever I'm searching for the right size socket and don't know if it's a metric or standard bolt, a quick glance to the table will tell me which wrench to try next. I use it a lot. It's a real time saver and could've helped in your case where standard and metric are nearly equal.
95 E320 Cabriolet, 131K