I personally don't view sewers as being to terribly bad. That is, unless you get lost in them. If that's the case, you might as well call it a life and find a good corner to give up in.
Otherwise, I actually think that sewers would be a great place to hide temporarily, being that there aren't too many living humans venturing down there on a regular basis let alone a single zed who just managed to stumble into an open drain.
Not all sewers are made for walking arround in. Out of your house it goes into a 4" pipe, hits the street into a 10 or 12" and into a 24" main. You'd have to live in NY or SF to find one big enough to walk in. Then theres light, sewers don't have them. Smell and bugs are another thing to consider. I'll take my chances topside.
in search and rescue when you are looking for little kids you start from where they were seen last and work down hill from there or toward the nearest loud noise I am assuming zombies will be the same so the higher up you are and the quieter you and you suroundings are the better
In my town a small town if I may add you can walk upright in them for some strange reason anyway I think they'd be great for moving around the city because your out off site and the sewers usually if not always follow the street so if you can get a good street map you'd probably be able to move around without getting lost
I would leave the sewers to the professionals with their knowledge of layout and innate dangers. In a time of dire need, I am not above hiding in a cramped, dirty, cesspool. A good Z.A. plan should shield you from places that dangerous though. This is coming from a mid-sized city's perspective. Our sewers are not cavernous mazes with shortcuts and man-sized access throughout.
sewer are a safe place if you we're running from cops or snipers
but it's not a safer place from zombies, maaan !
It's filled with diseases and it'll be a perfect zombie cave
and you won't fuckin' go there unless you wanna blowing it up
for good !