Tubes are sized the same as the tires, except that usually there is a slight range of tire sizes that one tube size will cover (because tubes can stretch in inflation). The manufacturer will include this "range" of sizes in the specs for their tubes as well as printing them on the carcass. As well, there are two methods of listing tire sizes: one is as you quote (the newer method, e.g. 110/90-18) and the older method (e.g. 3.50X18). These two methods have equivalent sizes (not certain if what I've quoted are equivalent); but, you usually want to stick with the same for tire and tube to be sure of a good match.
To answer directly your question: yes, go by the same size as the tire for your tube.
