I use one as it saves fingers when doing a stack of rqts, especially at tournaments.
I tend to use it once I've strung about a 1/3 of crosses, and then at regular intervals, but I also try and keep the strings straight while stringing.
It's best to push the strings rather than stab at them.
Head IG Prestige MP; USRSA MRT, Wimbledon stringing team since '04; Babolat stringing team '10-'13. Twitter - @therqtstringer