When I marshall I always try to get to cars in the order they stuffed it, though it doesn't always work out that way
The only exceptions being if there's a car in a dangerous place (eg middle of the straight) or if I've taken another car out on my way to the first crash, then they get priority. Also if someone blatantly runs into the back of someone else I make sure they're away last
So long as people put the effort in I'm happy, some people are physically more/less agile but if they're doing their best than that's all anyone can ask really. There are one or two people I can think of who don't seem to give a stuff when marshalling (no names mentioned...), that annoy's me as usually they are the first to complain if a marshal takes a little while to get to their car...