Testing is vital during early stages of a tank, like when you set it up! But after ammonia and nitrite read zero and nitrates are prevalent the only need is to test your nitrates to know when you Bio load peaks nitrates (to your max) thus a water change is required in order to reduce toxicity! Thus good for calculating a realistic and economical wc schedule!
Most experienced FK's stop using theirs after a couple months or so, some not at all. You can smell ammonia and obviously if your stock is decaying/dying testing is necessary to figure out the problem and a sure sign that a parameter is off.
It doesn't hurt to pull it out, dust it off and check you nitrates, nitrites, ammonia and even ph just for the sake of using it/peace of mind
But if you have a planted tank testing is vital at certain stages of the tank, even with seeded filters. Even a ph test kit can be used to gauge the ppm of co2 in the water column, and obviously once the plants are established it is necessary to know how much nitrates they are consuming because without nitrates a planted tank lacks 1 of its 4 critical nutrients (N-P-K & TRACE) especially with lightly stocked planted tanks!
And the same applies to other setups, like drip systems, initial dialing in is needed to achieve specific parameters!
So it all depends on your setup/s but the most note worthy "test kits" are your fish themselves!
Always advisable to have one on hand in case of power outage or other failure to check for mini cycles or other spikes. Definitely a beginners friend. And even a pros because testing is never a bad idea!
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Cheap way to decrease nitrates and keep your fish healthy: http://monsterfishkeepers.com/forums/showthread.php?t=504763