The current HIRES detector has 15 micron pixels which project to a small enough angular size on the sky that binning is recommended. For some choices of slit width, it may make sense to also bin in the dispersion direction as well. Binning also results in faster readout times. Without any binning the detector reads out in about a minute.
Detector pixels project to about 0.12 arcseconds on the sky, in the spatial direction. It makes sense, therefore, to bin by at least two in the spatial direction. For good seeing of 3/4 of an arcsecond, this will give good sampling of the PSF (i.e. 3 super-pixels) and reduce the readout time to about 40 seconds. Binning by 3 in the spatial direction will still yield adequate sampling of the PSF in this example and hence some observers choose this option which reduces the readout time to about 30 seconds. Some caution is necessary though when choosing to bin by 3 in the spatial direction. On nights of really good seeing, binning by 3 will undersample the PSF.
HIRES slits are available in a variety of widths and heights. Some of the widths project to more than 4 pixels so some observers choose to bin by 2 in the dispersion direction. One can select this option for slits of 0.86, 1.15, and 1.72 arcseconds width without degrading the resolution. Doing so, results in those slits projecting to 2.4, 3.2, and 4.8 super-pixels in the dispersion direction. Binning by 2 in both directions (2X2) yields a readout time of roughly 24 seconds.
When the HIRES software is first started up, the default is 2X1 binning. A discussion of how to change the binning (among other things) can be found here. The four most popular choices for binning are offered as hardwired selections on a drop-down menu on one of the HIRES GUI's. These options are 1X1, 2X1, 3X1, and 2X2. Setting the binning this way will ensure that other parameters, such windowing and overscan region sizes will be set correctly.