I just use levels and a gun vise which generally do an "ok" job and then "tweak" if necessary to get crosshairs so they appear level when I shoulder that rifle. I also like to use a plumb bob to check the vertical alignment, not fancy but seems to work for me. I've never had any problems when sighting in at 100 yards or when hunting.
If the hairs are a little off, should not be a big deal but if it is noticeable to you when looking through the scope, you may tend to tilt the rifle to compensate so they appear to be aligned which can change the POI. If the hair alignment is way out of whack, a horizontal adjustment also causes vertical shifting in the group and cause some grief when sighting in.