With stringed orchestral instruments, I've always thought of vibrato as a pinch of salt you add to a dish. Typically, you don't pour a full shaker of salt onto your food; you might, however, season it sparingly with a little salt. The same should be true of vibrato. When it's appropriate to give a passage added warmth, wobble the pitch as needed. If you use vibrato on every note, however, the effect is compromised, and it might look like you're showing off.
After reading about how vibrato used to be perceived in David Byrne's impressive book, How Music Works, I felt vindicated about using it sparingly on the upright bass.
