Well first off, wood is a natural product and it will show cracks over time. By keeping up with the maintenance you will be able to extend the life of the wood deck for many years.
One of the biggest catches is that while the sealers may show 3-5 years life on the cans that generally only applies for sheltered or protected decks. Those seeing a lot of sun, use, or other issues will need treatments far more often.
This is especially an issue with today's fast grown wood as compared to old growth wood with tighter grains. Minute cracks are generally not an issue. Cracks where the wood starts moving are a big issue.
Make sure you also don't have water issues under your deck, you don't block air flow and you keep it clean.
For more information:
Read Sean Lintow's blog post "Sealing your deck properly."