Not necessarily, just make sure that you get a tire with a low enough profile. You basically want to keep the total diameter almost equal to the stock wheel and tire. You can use this calculator to figure it out: http://www.rims-n-tires.com/rt_specs.jsp
18" rims need either 215, 225, or 235/40-18 tires. Because manufactures vary slightly in size in width, I would suggest 225/40-18 tires to be safe. Also , you need to have the proper offset which should be around +45mm depending on the width of the rim. Since you did not tell us what car you have, if you have a LX with drum rear brakes, make sure the rim will clear them. If you have a hard time understanding what I just explained, tirerack.com has a awesome technical section.