Indicator Panel Method
In Valve's Portal 2 maps, they appear to use func_brushes with the signage/signage_doorstate texture on the front, changed using an env_texturetoggle for all of their indicator panels. They're usually incorporated cleanly into the surrounding tiles.
The other way I've seen it done is with a prop_indicator_panel in a static prop frame, as described on the VDC. The frame is also used in all of Valve's Portal 1 maps.
Which method, the func_brush and env_texturetoggle way, or the prop_indicator_panel and prop_static way, is more acceptable in Portal 2 maps?
I would use an overlay, though, the indicator panel frame is not used in Portal 2. Valve uses an overlay then an env texture toggle, though I may be wrong
I've just been slapping a doorstate overlay onto surfaces so I can refer to all of the light related entities with the same name. Valve usually didn't use the frame in Portal 2, so I don't unless I think it would make it look better. I don't know that any way is more efficient than another, unless of course you plan on using a timer. Personal preference, maybe?
Well, prop_indicator_panel doesn't require a env_texturetoggle (like BOB74j said), making it a little easier. Also, considering the mistake of the check on a func_brush appearing backwards if not made or edited correctly makes me use the prop_indicator_panel even more.
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Up to you, really. if you prefer it with a frame, use one. if you dont, dont. personally, i think it depends on the context as to which one looks best.
I think in terms of boolean variables. Generally, it makes things easier.
I personally use the prop_indicator_panel without a frame. That way I can use the advantages of the entity, easily move it and still have the typical valve look, as this entity has no visible frame.
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I looked at the example maps, and they actually used func_brushes along with an env_texturetoggle.
I think I've found my new favorite method.
Oh, so that's how that works! I remember playing a few maps with backwards check boxes when I just started using Hammer, and I couldn't wrap my head around how people had managed to do that.
That's actually what I've been doing as well, but I looked into how Valve did it after I discovered that, without a frame, indicator panels can't be perfectly flush with the wall it's on. It's either on the same grid line as the wall, in which case it flickers just like overlapping brushes, or it's just one unit off the wall, then it's very subtle, but still not quite right.
Make a hole in the wall and cover it, then it's perfect.
I prefer to use the overlays. That way I can just use the same env_texturetoggle to toggle my indicator lights and indicator panels! with just one output 
~lp
