Yep, all part of the service.
There are rules though. They have to be good people that I like. Also, they need to be prepered for me to land on them once in a blue moon.
I find it usefull for days when I get a kickback from messers, I can call in and do the job and rely on it in an emergency.
It is only something that can be done as an addition to an established round.
Most of the "as and when" jobs that I have are favourites that I have hit it off with. Sometimes you click with people on a personal level and they are more like friends than customers.
They are a useful addition to my round.