Hi. It depends on the situation, for me. I have two meters: a Pentax Digital Spotmeter, and a very early version of the Sekonic Studio Deluxe called a "Brockway", for the 1950s. I usually have both meters in my pocket, though I usually only bother using them if I am pretty unsure about an exposure, or if I have the time (which I usually do not).
For more formal pix, I use both to decide an exposure.
As for in-camera meters, I use them extremely rarely. I usually knock the meter into the on position and run down the battery anyhow, so they don't jive with each other camera to camera. I trust an educated guess more than my particular in-camera meters. I do use them occasionally as a last resort double check of my guessed exposure when I know for sure that they are working properly, via a grey card test. My Brockway and my Pentax almost always jive within a half or third stop. Every light meter is a bit different, and if you are calibrated to one, using another one can screw you up.
I don't have a flash meter. I probably need to pick one up pretty soon, however, as I will be transferring to a school that will make me do a lot of in-studio stuff with flash.