Comic Care: How Reliable Are Poly Bags

So, I've been reading for about 3 years now, and I keep hearing how poly bags need to be replaced every 5-10 years. However, others have said that it was bogus. Although I do plan on getting mylar sleeves for my favorite and signed books, I want to know what the Vine's opinion is on this. Do poly bags really need to be replaced or is it just a lie to get more money.