No, not true. The quality of the actual image will be the same on a £2 memory card and a £30 one. The thing that varies is the amount of time that the data can be stored without degredation of quality.
A better quality memory card will be able to store the data without loss of quality for a longer time than the £2 one will be able to (it will prob develop bad sectors and become unreadbale in an extreme case).
That answer your question........?