[MWillner]

How seriously do people track noise and background count? Do you make a note of it for every encounter?

How uneven are they? Can you walk a mile and have them change several times, or is all Seattle basically at level 0, with only small areas being off the baseline? Are the boundaries gradual or is there an abrupt change?

Thanks for your ideas.

[The Wyrm Ouroboros]

Astral space is never going to be even - but from a GM's perspective, almost everywhere not toxic (I'm looking at you, Glow City) is going to even out to a general zero.  You can walk a mile and have them change several - even many - times; background count can bump up from powerful magic, extreme violence, very powerful emotion, etc. etc.  However, it really should only change as often as the GM finds it interesting/necessary - maybe just to the side of where the 'runners are investigating is a cult's 'holy site', and so the Count is up by a point or two; the players don't need to know this, but the GM does.  Or the scene they're going to investigate is up by a point, because of the horrific murder that happened there, and that the PCs are going to stumble into the crime scene of.

Essentially, use the Laws of Storytelling; make the 'count vary, because it always will.  Only in specific areas (ley nexi, concentration camps, places of long-term worship) is it going to go up and stay up for a long enough period of time for you to have to indicate it in permanent marker on your map.  Otherwise ... just use dry erase. 

[SirValeq]

You can also use the background count and noise to level the playing field for your PCs in action. Visibility modifiers are easy to stack and can hinder your combat specialists. If you don't want them to feel less useful than the rest of the team, introduce noise and background count with a plausible reason.

[Walks Through Walls]

As was already said the variance in noise and background count doesn't need to be constantly tracked, but can make a great story telling tool. It can also be used to keep the players off balance. The occasional spike can make them wonder why or hinder something important they are about to do. It needs to make sense. This can be fairly easy but just thought of ahead of time. So examples could be.

Noise level of 1 or 2 downtown during rush hour and the spam bots are broadcasting full tilt trying to lure anyone and everyone in.
Background count of 1 a recent clash between the Ancients and Holloweeners left five dead including two onlookers.
Background count of 3 inside the hive of the ant shaman that is about to summon up the queen spirit.
Noise level of 3 in the basement of a lava buried church in the Polyup.

The possibilities are endless. Just think about what makes sense and improves the story.