They are most likely right, but no guarantees. The creosote will build up where it condenses and hits the chimney wall. While it usually happens in the first portion (from my limited experience) the actual answer depends on the temperature of the fire, the temperature of the flue, the draw of the flue and this varies along with the outside temperature.
In my case, I clean the entire chimney once a year, but I do the bottom five feet about four times a year since it is very easy for me to do. Your house will certainly be a bit different.