When it comes down to it, there are probably quite a few insurance companies who wouldn't give a rats fat one if your vehicle, whilst under warranty, went to one of their 'approved' accident repair centres. They obviously do this in the first instance, to save themselves money. You will never be sure that the insurance company 'approved' service centre has used genuine parts when fixing up your trashed motor.
I agree, when it comes to renewal, the insurance company you've been with might want to somehow punish you by whacking up your premium because you didn't play their silly little game of doing exactly what they wanted you to do at the time, but in its early years, whilst the vehicle is under warranty, it's worth having it repaired to Fiat's approved standards, not some bullish insurers. I personally think that the service that Fiat are offering, is worth something in the event of an accident.
I perfectly understand, that for younger drivers, insurance premiums are an absolute joke and you will of course want to do everything to keep them down. I'm not sure how this compares to say a 20 year old, but my wife's premium for the year on our 1.2 POP, was just over £200 fully comp, so us sticking the middle finger up to our insurance company and letting Fiat do all the negotiating with our own insurance company in the event of an unfortunate prang, is hardly going to cause us too much problem at renewal time. And of course, how many people these days actually stay with the same insurer year after year? I for one, change mine nearly every year, just as I do with my buildings/contents insurance.