Use of this technique can reduce distortion of line profiles and bring out features otherwise lost due to poor charge transfer. Obtain 20 - 30 per night. Can be spaced throughout the night or split between beginning and end of night. Zero exposures are important for the TI CCDs where preflash is generally used, but may not be necessary for some of the TEK CCDs where the bias level is very uniform. In this case one could use the overscan or the region on each side of the spectrum to subtract the bias level.
Dark integrations are probably unnecessary for most applications.
You may want to obtain a couple 10 or 20 minute darks just to check on dark current and look for any hot pixels. The dark current is sometimes high if the chip has been exposed to too much light, or if power has been interrupted to a cold chip. The spectrum may be moved to avoid any bad regions on the chip. Obtain a total of at least 10 times the number of ADU's compared to the count rate of your most intense spectrum. On-chip summing will extend these limits somewhat, but caution is advised.