It is illegal for the manufacturer to require you to go to their dealer for maintenance during the warranty period. You don't even have to go to a mechanic. But you better have well documented records of doing the required services on time and with approved parts to make a warranty claim if you don't.
One argument for going to the dealer is that there have been rare cases where going to a dealer who sold the car and had serviced it and the owner has records of all that has proved beneficial in a slightly out of warranty situation. The dealer can (but doesn't always) go to bat for the owner and gets some special consideration on the price of the repair. Since you have a 2008, you may want to consider this.
Everyone must make their own decision on the expense versus the possibility of reward.