I don't think I must understand what you are saying.  

With the head on the ground, the accumlator is empty of oil and the nitrogen is expanded to fill it.  As you activate the valve to raise the head, it raises the pressure in the oil line until it is high enough to overcome the weight of the head.  The head goes up.  On our combines this occurs at about 1500 psi with the flex head on.  When the cylinder is fully extended, the cylinder stops moving.  The pump will raise line pressure to the relief setting, pumping oil into the accumlator and compressing the gas until it reaches about 2800 psi.  At this point the head is more-less rigid in the up position and the accumulator is prety much full of oil, with the nitrogen compressed into the small remaining space.   If you hit a big enough bump, i suppose you might spike the pressure a little bit, but not much, and you aren't going to get much cushion out of it.  If you stop holding the valve as soon as the head is all the way up, before the pressure goes up, and leave the pressure at about 1600 psi or so, you will have a very nice cushion of nitrogen for the head to ride on.   

The only way I can see what I seem to be reading you to say is happening is if you have your accumlator charged to a pressure much higher then what is needed to lift the head, say for instance 4000 psi, in which case it would remain pretty much empty of oil but would provide practically no float when the head was on the ground and the pressure was low because there would not be enough oil pressure to push oil into the accumlator to provide upward travel in response to floating on the ground.