When to use f and when \nu signifying frequency?

http://en.wikipedia.org/wiki/Frequency

For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time. In physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by a Greek letter ν (nu).

I usually make "usually" bold for personal pleasure which usually has nothing to do whatsoever with notation convention.