Answer: Generally speaking, seed viability decreases over time and results in decreased germination, regardless of storage conditions. Keeping them cool and dry is about the best you can do; the refrigerator is a good place to keep them. In addition to keeping them in sealed containers or jars you might consider adding some silica (such as those packets occasionally included with new cameras or leather goods). Be sure the seed envelope is also dry when you put them into storage. Finally, be sure to label and date each batch of seeds and keep a log reflecting the historical germination rate of the various types. This will help you predict when to replace your seeds.
Tomato seeds are particularly long lasting and so a few might make your 5 year mark; most others will not. Here is a chart http://www.colostate.edu/Depts/CoopExt/PUBS/GARDEN/07221.htm...
of vegetable seeds and estimated storage times; frankly I would use it only as a rough and optimistic guide because these numbers are based on professional storage techniques. (In addition to home storage conditions, you have no way of knowing how the seed was handled prior to purchase.) I would also caution about the use of heat to dry them: without the tools required to measure the moisture content a home gardener has no way to know when the seeds have reached an overly dry state.
The only way I can think of to attempt to ensure a continuous supply of seed over any length of time is to grow open pollinated varieties and save your own seed from year to year.
Q&A Library Searching Tips