It depends on the brand - some let you start the warranty at installation, some on date of purchase, and some on date of delivery. I would call the manufacturers to confirm their policy. It's a good idea to have some proof of the installation date.

SUbzero/Wolf has that policy.However their display warrany is about half the length of their regular warranty. I think I have hit a record of 5 years since buying and just confirmed through authorized installers (they gave subzero the serial numbers) that the warranty will definitely start on installation. BTW if you get it installed through authorized installers Subzero/Wolf will tack on an additional year of full warranty. As weissman said this is easy enough to confirm through the manufacturers but you may need the serial numbers.