The reason for the general statement is because electrolytics are leakier than other types, not as stable with temperature and time, etc. So, if you need precise, repeatable timing in a circuit, you would try to use a more low leakage, stable capacitor type - like a teflon, polystyrene, polypropylene, etc. The problem is that these caps are generally not available in large capacitance values. However, a 555 is not a precision chip, so it is not used in precise timing applications, so the use of non-ideal capacitors (from a leakage standpoint) is usually not a problem.