Usually if you do a wc once a week (or two) you never really need to unless you see something go awry. I never change H2O for a few days after a heavy rain, I've seen too many tanks crash around here when people do it. After testing customers' H20 for over 5 years, I got to where I could predict most people's readings after asking a few questions (and getting truthful answers).
If you follow a few basic rules, you shouldn't ever have chemistry problems unless your supplier is whacky.
The most common problem is when people zap their bioload and keep feeding their fish like it's going out of style, or they never change water at all (and feed like it's going out of style, usually).
When I go to my clients' houses and clean tanks, new tanks always get a test first thing so I can see what I'm dealing with. My old customers only get tests if they need the reassurance. I treat my regulars' tanks like my own, and pretty much know everything that goes on with them, so I don't worry.
However, in case of disease, water tests are the FIRST thing you should do. 95% of the disease I've seen is directly related to poor water, and 95% of those are due to poor husbandry.
