Is a matching network needed if the antenna feed line is very short?
Do not assume that you do not need any matching network if the line is short. 

Your circuit which is connected to antenna, probably is designed for certain impedance. If antenna itself doesn't have it, your circuit probably doesn't work optimally. Radio transmitters even can get overheated or generate overvoltages with wrong antenna impedance. Radio receiving can be too noisy or have wrong filtering if the antenna is mismatched. 

Your 10% rule makes the transmission line matching not so important, but it doesn't remove the need to match the antenna with the rest of the circuitry.

Is that need existent - the designer of the circuit knows.