I am not certain that I understand what you were told. There is some reflection issues with optical cables that are very short and a cable that is perhaps 2m would be better than a .5m cable in that regard. It is more a matter of suceptibility to jitter on the side of the DAC. Toslink (I am assuming that is what you have) cables only have a useful transmission distance of 5m, so you can't use one much longer than that. If you don't notice any problems, you should be OK. If you have occassional drop-outs or unsatisfactory sound quality, try a 2m or 6' cable instead.
-Bill