In my more paranoid moments I have been known to clip a crab through the holes at the top of my handled ascender which stops the rope escaping when I have been using it for protection or assistance on steeply sloping traverse line or similar, or when rigging/derigging.
from your description this sounds like proper usage of the equipment (not paranoia). assuming the old-shape handled petzl basic jammer this would be standard practice as required by the manufacturer to avoid danger of death.
https://www.petzl.com/sfc/servlet.shepherd/version/download/0681r000004z4JCAAY
note the current-model 'small' basic jammer instructions differ here but still require special action on sloping ropes.
https://www.petzl.com/sfc/servlet.shepherd/version/download/0681r0000078a2AAAQ
this might also be of interest:
http://www.scanrope.eu/download/ascender/AscenderReview.pdf
lots of interesting and useful discussion here - particularly interesting to hear users' experiences of jammer failure (there are more than i had anticipated).