I believe it depends on the type of transformer. Some modern transformers will not turn on with the throttle in the up (on) position. This is for safety and protection. They do not remember the last setting.
Both the CW-80 and MTH Z-1000 have marked graduation lines so that you can remember where the throttle has to be to achieve a particular voltage level. More expensive transformers have on-board displays to let you know what voltage you are dialing in.
You can always put a sticker on to remember where 18 volts is. On my old CW-80, 18 volts was always with the handle perfectly straight up and perpendicular to the base so that it only took a second to set to the correct voltage.
The LC/+/2.0 engines are not overly sensitive to voltage. Mine always ran ok anywhere from about 15-18 volts so, if you're off a little on the down side it should operate fine.
