Watering plants seems like a compulsion for humans, often with total disregard as to whether or not the plants actually need water. To paraphrase Mike McGrath, many more plants have been killed by overwatering than by drought. Once a seedling has sprouted, it is usually beneficial to allow the plant's soil to dry out between watering. This stimulates root growth as the plant reaches out for every last drop of available water. Another good reason to allow roots to dry out is to avoid root rot. Most vegetable plant roots will eventually rot if constantly immersed in water, especially in poorly drained pots or soil. It helps that seedlings you buy from a professional nursery live in a well-drained plastic pot of adequate size with the right sort of potting soil for that variety--chalk one up for starting with seedlings your first year of gardening.
So the question remains: When to water? In our experience, the best way to judge a potted plant's soil moisture is by weight. Obviously, wet soil is a lot heavier than dry soil. A good way to observe this is to fill up two small pots with soil and give one of them a good drenching. When your plant weighs about the same as the pot of dry soil, it's time to water. Another good indication of water deficiency in plants is wilting. While it may seem like a drastic decline in health, wilting is simply a natural process that plants employ to pull water out of the leaves and stems and store it down in the roots during drought periods. While I usually try to get my seedlings watered before wilting occurs, I've found that it's not a big deal at all for well-established seedlings to wilt a bit.
Contrary to popular belief, the watering can is not the best watering method. The best way to get water to the plant roots is to use a bottom-up technique. We put seedling pots in a tray filled with a couple inches of water, then the water is pulled through the holes in the pots (your pots do have holes, right?). Don't worry if the surface of the soil never gets all that damp; the soil will be thoroughly and evenly wet where the plants need it most. Bottom-watering is a good habit because it's essential when starting plants from seed. It's also a good way to keep your plants' leaves dry; wet leaves are an open invitation to all kinds of airborne fungus and mold spores. While it's fine for the leaves to get wet, standing moisture at the soil is an invitation for fungus colonies, especially for crowded plants.
For outdoor plants, the watering can is actually more effective than a hose, since you can direct the water to the base of the plant's stem. While a garden hose may take less time, it really doesn't do a very good job of getting the water where it needs to go.
Don't go throwing all those plants in the ground just yet. Up next are Timing and Supplies, and then you can fly free. In the meantime, ask your questions, people! We know you're itching to.