I can't give a direct answer to the coking question however I previously worked for a company providing helicopter support to the UK police using AS355N and used AS355F1/F2 as back-up aircraft. About nine years ago we were directed by the company not to use any remaining aerosol canned stocks of PRIST ourselves at point of refuel, because it was too toxic. If my memory is correct the N didn't need PRIST but the other models did. It wasn't a problem to us because the supplied bulk fuel at base had FSII aditive but I don't know how other units dealt with it.
The two types I've flown since don't need it as they have fuel warming in their systems; the RFM being the documentary authority I refer to.
It's the fuel temperature that counts; will your Astar sit out in freezing temperatures, getting cold soaked? If so, surely the additive is needed. If the RFM calls for it, so would the insurance company. When making aviation decisions, I always ask myself "what would the board of inquiry say?"