all I use to water my orchids
Improper watering has probably killed more orchids than all other forms of neglect combined, and was the likeliest cause of death in my prior attempts at growing orchids. Unlike most houseplants, orchids need to have their roots dry out between waterings. Otherwise they rot. Often by the time the leaves of an orchid start to look withered, the roots will have already rotted away. The dehydrated appearance of the leaves only adds to the confusion, when over-watering caused the problem in the first place.
Most orchids are planted in bark mixtures. Unlike potting soil, orchid bark mixes don't exhibit a capillary effect, so a dry mix on top can still be very moist just one layer deeper into the pot. (This is often all too true for sphagnum moss as well). Orchids benefit not from the wetness of the media, but from the high humidity that the wet media helps produce in the air pockets within the pot.
fast-draining media is crucial for orchid root health
All my plants have either a wooden skewer or a plastic stake in the media, which I use to check for dryness. I water at least some of my orchids every 2-3 days, although individual plants can go over a week before they need watering. The temperature in the apartment, the room humidity, and how actively an orchid is growing all affect its water needs.
when the bottom of the stake is dry, it's time to water
I allow my cattleya to completely dry out between waterings. The plant currently has a healthy root system, and plump pseudobulbs on its new growths. If the plant doesn't get enough hydration, I will notice a wrinkling of the pseudobulbs and will be able to adjust the watering frequency accordingly. With this one, I'm really trying to err on the side of underwatering, rather than risk damage to the roots.
round shiny pseudobulbs indicate a happy plant
I keep my 2 phals slightly wetter than the cattleya--I water them when the skewers are mostly dry, instead of waiting for them to fully dry out. I've noticed that the big phal seems to only need watering once a week or so; the bigger pot takes longer to dry in the middle.
For all my other orchids, I wait until the skewers are partially dried out. It's harder to describe this level of moisture, but it's somewhere part way between freshly soaked, and completely dry. The paphs and oncidiums have thinner roots, which like to have a constant level of humidity around them. The dendrobium victoria reginae is a wet-growing orchid (unlike most other dens which prefer dryer conditions). Edit(1/18): due to the declining health of the dendrobium, I have decreased the watering to paph level. Lastly the cymbidium is in full active growth, and using up a lot of water as a result.
Since I don't know the quality or hardness of Manhattan's water supply, I filter my water through a pitcher before using it on the orchids. I find myself needing to replace the pitcher filters every few months (or else they slow down to an infuriating trickle) so the filters must be catching something. Some orchids are more sensitive to water quality than others, but I honestly don't know if mine would fare any worse with unfiltered tap water. Ultimately, filtering is easy enough that I err on the side of caution.
To water my plants, I place each pot into a bowl or drainless ceramic pot, and fill it with water up to the level of the potting mix. I then allow the plant to soak for about 20 minutes, before I drain the water from the pot and replace the orchid on its shelf. I reuse the same 3/4 gallon of water on all my plants.
The only exemption to this process is my angraecum leonis, which I grow in a vase rather than in potting media. Here, there is no risk to rotting the roots, as they are completely open to the air. I water my angraecum anytime I water any of my other orchids: every 2-3 days. Edit (1/18): I now water my ancm leonis daily. I simply fill up the vase with water up to the very top, and let it sit for about an hour. The roots turn bright green as they soak up water. Then I pour out the water until none of the roots are submerged, but leaving a small amount in the vase to add humidity.