From my experience, the brightest spots vary from 85 to 90IRE in Winter and somwhere around 80 to 85 in Summer time.
A 75IRE highlight would most likely look a bit too underexposed to me, unless we're filming Mexican or Middle East people.
But, again, it all depends on how much zebra you allow to be visible in the hot spot.

Personally, I tend not hang up on the levels that much, I tend more to use a correctly set up CRT viewfinder or monitor and use those to set the exposure to give the look I want. Where the zebra level comes in and how much depends on a number of factors including skin tone, skin texture and the lighting. If the zebras come on I know I'm in the ball park, even if only on the back of neck, which can happen on a backlit scene, but sometimes they won't because of the subject has a tan. You can't be slavish, you have to use your eyes.

I've seen people overexposing material because they think the 95% bars are 70%.

Sorry guys, I should've clarified I meant run and gun situations. Of course, if you have a monitor, screw zebras! If you're running around with a cam trying to shoot a million different things with a million different light levels, this is where zeebs might be handy.

I agree with Bryan that zebras are only ONE of the available tools. The other situation I ran into was on a rental Betacam a number of years ago: I trusted the viewfinder (from my local broadcast rental house) only to find out later that I had over exposed just about everything by a stop because whoever had used the camera before me had turned down the brightness on the viewfinder. Again, this is MY fault because I didn't think to throw bars up and set brightness using the pluge because I was already half an hour behind schedule... Live and learn (usually the HARD way...). Now I NEVER make that mistake AND I own my own cameras...

This is the whole point, Shaun. You never know who used the camera before you and how the viewfinder was set up. Zebra is a help here.
Back to the question - we have two cameras in the studio with either 70% or 100% zebra detect only (all other cameras we own have increments of either 1% or 5% so you can practically setup any zebra threshold you want and we have them all at 85%), so we found 70% absolutely useless and simply turned zebra off on those two cams, and I was qurious too - what that 70% zebra is needed for? How do people use it?

Well Stas, I don't know what to tell you. Based on your explanation of how you're exposing and my experience, your images will be SIGNIFICANTLY hotter than mine, if you're using 85 IRE as reference for skin tone highlights. I get by perfectly with 75 IRE and can deal with 70 IRE.

I guess this just goes to prove that there are MANY ways to use the same tool.

My only question would be: are you preparing video for broadcast or for web or DVD? If you're preparing for broadcast, it still sounds way too hot to me but I'll assume you're doing the quality control necessary for your distribution medium. With highlights at 85, there's not much latitude left before you get to 100 IRE broadcast safe white. UNLESS you have very shiny skin to deal with.

I think this will depend on on the type of camera you're shooting with. Using a 2/3" camera with a good CRT V/F you often don't need them, but you can work with 70% if need be. Shooting with manual exposure on a prosumer camera with a poor LCD V/F you a reference of some sort does help, although setting the aperture can be fiddly due to the 1/4 - 1/2 stop iris increments, plus the lag in the system. Which zebra you use does depend on the subject matter.

Anyway, these cameras are so fiddly in fast moving situations that a quick tap on the auto exposure to set stop could be the best method. On the 2/3" cameras the ergonomics allow you to stay on manual exposure no matter how fast things move.