Is the black wire always hot on a three-way switch?
Basic 3-way switches
When working with basic 3-way snap switches, the switch should have four terminal.
Common, COM, or C
This terminal will typically be black, or some color other than green or silver. It may be brass, in which case it will be labeled or marked in some way as common.
The common terminal will either be connected to the ungrounded (hot) LINE wire coming from the power source, or to the ungrounded (hot) LOAD wire going to the load (lights).
Ground, Earth
This terminal will typically be green, and will likely be either at the top of the bottom of the switch.
Travelers
These terminals will commonly be brass. These terminals are used to carry power between the switches, and will alternate being live.  Depending on how the switch is toggled, one of these terminals will always have voltage (assuming the circuit is working properly).
Smart switches
These types of switches will usually offer some extra functionality, other than simply turning the lights on/off. Because of this, the switch may also require a neutral wire.
Neutral
The neutral terminal on a smart switch will often be silver, and is used to connect a circuit neutral wire to the switch.

Wire colors
Wire color is meant to help determine the function of a wire, however, it can often be wrong.  White wires can, from time to time, be used as ungrounded (hot) wires (though they're supposed to be marked when they are).  Your best bet, is to never trust the person that installed the wiring, and always test the wires yourself.

3-way circuits
A 3-way circuit should look something like this.

3-way circuit
However, wiring in a building is not typically this straightforward. So you might find cases where power goes to the light first, or other strangeness.