Re darks:- With my extremely faint supernova spectra, I no longer do a traditional dark correction though i still take them to produce a defect map and use this with the bias frame. I have not taken that step with other spectra yet though. I use cloudy nights to take darks so I have plenty of opportunity to take very large stacks !
re flats:- I always take and use them with slit spectra as I find it is the only way to accurately remove some fine scale QE variations (QE ripples at low resolution and interference type effects at high resolution). Getting a big enough stack to avoid introducing noise without getting saturation is a problem with a halogen flat source and wide spectral range though. (eg ALPY). Again cloudy nights come to the rescue here.
For slitless work I have come to the conclusion that flat correction is effectively impossible so just take a flat so I can avoid any obvious defects and position the star zero order (target and reference) in the same place in the field.