Some people will tell you that it doesn’t matter, while others who abstain from ‘choking the chicken’ may indicate a two to three hour window both before and after. The idea is that once you ‘release’, you lose a lot of the engorgement in the penis from all the blood flow. This of course would seem to be important in the process of jelqing.
If you had to do it, I would say wait until afterwards. At least you’ll have completed your routine for the day. Besides, if you do it before, then you may have a harder time maintaining an erection. I suppose if you were stretching, jerking off before hand would be a good idea to ensure a total and complete flaccid stretch.
Just my opinion…