I agree with that totally. The first service is the most important as far as warranty goes and it does establish a relationship with the dealer.
I also have never heard of a warranty issue being denied because of maintenance records, but it pays to have the dealer on your side.
BUT , I have heard of bad dealers with bad mechanics, usually multi-marque dealers who have the Triumph franchise but no real interest in supporting them.
If you know your way around a bike, check it over after the first service. Get to know the dealership's owner, the mechanics, the parts people, and make your decision....
And as mentioned, keep records and receipts.
