~ Follow Nature
Nature is good at regulating itself and a good thing to follow and pay attention to.
If it rains outside water your indoor plants, they'd be getting a drink if they were outside in their natural environment.
If it rains for more than one day give your plants more water, but not too much. A good water on day one (generally around half a cupful of water for medium plants, depending on plant and pot size), lessening the amount each day depending on how big your plant pot is.
~ Heat Waves
If you had a really hot day where you've just about died of heat exhaustion and sat around all day because it was hot to move it's a good idea to give yourself and your plants a drink.
None of us can survive without water!
When watering your plants outside, use water you've collected from those rainy days in containers around the garden and from rinsing dishes. This saves on having to use water from the tap and is also a great way to use up those containers that gather water (and mozzie larvae).
P.S. dogs like to drink rain water so you're also looking after your pets.
~ More Needy
Concentrate on newly planted plants and those looking like they're keeling over. Good healthy looking plants can be ignored, up to a point.
Well established plants can generally go without water for up to 3-7 days without your watering them - or never in some cases. Generally well established plants are good at finding water sources themselves and don' necessarily need your help.
NOTE: If a plant died or is dying this means it's time to plant a new and different plant there: one that can thrive there, whereas the other one wasn't suited to that spot.