How early can I claim a train?

Hi! For the first time in a long while, I should be able to join today’s session right from the start time (hooray!). I was wondering, how early is it acceptable to claim a job from the work list? I thought it made sense to look at the work list ahead of the start time, find something that’s interesting to me and that I feel confident working, figure out any details I need to figure out, and claim it, so nobody else goes and takes it and leaves me having to back and start over at the start of the session, wasting precious time when a dispatcher is available. But, I also realize it might be unfair to others to claim a job way ahead of the session, if everyone else expects to just arrive at the start time and grab things then. So what’s the usual rule/etiquette for this on SWRS? This is specifically for the case of DS-required jobs (non-DS-required jobs I would only claim when I was ready to start working them, because I could work them any time and there are usually no other people around anyway).