Approach to Reverbs and Delays in the mix?

Do you tend to have dedicated reverbs and delays that you choose to send everything to, or do have separate ones for each instrument? For instance, does the vocal have it's own reverb(s) or do you like everything to live in the same space in terms of sonics?

Also, do you start the mix with those buses up and ready so you don't have to think about adding a reverb?