This is only my opinion, and many on here will not agree, but unless you can get the 5 year guarantee for free, and the set is the same low price as you can get without the guarantee, I would not bother too much about it.
Most electronic and electrical equipment tends to either go wrong in the first 6 months or not for many, many years. You already get a one year guarantee with most electrical products, so that covers you for the most dangerous time 1-6 months. Obviously you could be unlucky and the set might break down after 3 years. If that bothers you get a 5 year guarantee, but as I said, make sure it is free. I would never pay for a 5 year guarantee, as I would always take the gamble that the set will not go wrong between years 1-5. And if it does ,I would put the money I have saved, to fix it or towards a new set.
Add up all the electrical products in your house and see how much you would have spent on extended guarantees for all of them if you had paid. Then ask your self, how many have packed up before 5 years? Shops that push 5 year paid for guarantees are betting that most of what they sell won't go wrong between years 1-5.