Hi ezeiza, I'm assuming your question was directed at me.
Many years ago when I first thought of growing Hedychium here in Cyprus I sought the advice of Lyn Spencer-Mills who, at that time, held the National Collection of Hedychium in the UK. She cut her plants back to ground level every autumn, admittedly after the first frosts, a problem we seldom encounter at our altitude here in Cyprus. I cut mine down usually because following flowering the stems begin to die back and by the time the new growth starts to appear (April/May) they are falling over anyway. So, I guess it's a simple matter of aesthetics really.