I just read a some bad advice on a lawn care web site: overseed your lawn every few years even if your lawn is in good condition, and use a lot of extra seed because you will probably forget to water it often enough to properly germinate. What a waste of time and money!
Grass seed does not germinate without good contact with soil, so spreading seed over existing turf without preparation is an exercise in futility. To properly overseed a lawn, you need to first mow the grass very low and then rough up the soil, a procedure that destroys the existing turf. And, if you are not going to remember to water overseeded areas, don't spend your money on the seed and other materials needed to do it right.
If your lawn has bare spots or is looking thin, you can overseed the right way and save money by following the steps in Budget Lawn Care - Overseeding. If you live in the Midwest to Northern United States or Canada you will need to get started with overseeding right now or hold off until late in the summer.
If you are happy with your lawn, don't put your time and money into unneeded maintenance. Check out how to fertilize your lawn and maintain your lawn to find out what turf maintenance is really necessary.
As a master gardener with specialized training in lawn care, I have seen a lot of uninformed or irresponsible advice on the Internet. These articles are usually trying to sell a product, so do not be taken in by scare tactics like you will lose your lawn if you do not overseed often, and don't waste your money on unnecessary lawn care.