You want to test them out of circuit if you suspect they are bad
I generally test in circuit and if they read close to what they are supposed to then I leave them, but if they are a little off I will pull them out and test that way
But like steve said circuits vary, for example I was fixing a Bose surround sound which the LCD screen did not work at all, there were two 100uF caps, one read 89uF out of circuit and one read 90uF, replacing them with new ones fixed the screen, in some cases a slight difference can make all the difference