ILS Critical Area signage: Localizer or Glideslope?

Ok, since we're on the subject I have a question regarding this.

We are all familiar with the ILS Critical Area signage on the
taxiways, requiring you to hold short of that line to keep from
interfering with the ILS signals (glideslope or localizer).

I have always been under the impression that when you see those lines
you don't cross until your given permission too (cleared onto the
runway, etc.) However I'm currently working on my CFII right now, and
my flight instructor the other day told me that those areas are not
always active and you don't always have to hold short of them. He
wasn't real clear on where or how you'd find out when the area was
"hot" or not, which leads me to question whether that is true or not.

Anyone know a defintive answer on this?