I have a potential customer who is going to switch to geothermal. He already has a shallow well for his sprinkler system, but NO pressure tank setup. On a call the pump come on and shuts off after the call. Sprinklers work great. I understand pressure in the coil in the off mode helps fight mineral formation in the coil, but does pressure or lack there of effect efficeincy during operation. We are on the east end of Long Island, NY. Water is high quality (sandy soil) and there is a spin down filter for sand particles already on the supply line.

Do I need to add the pressure tank 

Any thoughts???

pressure is not needed for the heat pump to work,  flow is the only factor.  However,  keeping the coil under pressure is recommended for the reasons you stated.