I wouldn't repot my nepenthes each year as they don't like having their roots disturbed from what I've seen.
I've just repotted some of mine and quite a few of the older leaves have just died but the rest of the plants are still healthy.
Like Aidan, my ventrata was sitting in the same soil since I bought it from a garden centre but only for about 5 years. The soil was looking rank so decieded for a repot.
I have potted mine up in 100% pure sphagnum.