If the chimney is of fairly new construction with tile lining, and the mortar has not fallen loose, you might be safe from flue gases entering your home. There are many ifs in chimney safety.
A masonry chimney is usually of a fairly universal size for a home. If the boiler is sized to the heat loss of a home built in the last 20 years, it is relatively small and therefore may have less draft - the liner will improve that by acting as another heat barrier. You have less worry about condensing moisture in a warmer (than the outside) chimney.
So the choice is that you might get away with not needing the liner, or you might need the liner to prevent flue gas leakage into the home and create a better draft. Not putting the liner in is a gamble, installing the liner increases the safety margin. Your choice.