It's safe up to a point. My 6990's are below 80 most of the time and have been for months. 5850's stay 75ish. But I think more important than just the heat is the electricity usage and fire hazard.
I have a rig that pulls 11 amps when mining. I purchased one of these Kill-a-Watt's
and tested each of my setups first. Then I went to my breaker box and figured out how many circuits there were and if I needed to move anything to another outlet.
Turns out my 11 amp rig is on a 15 amp circuit. So I moved everything else off it and I am just below the 80% recommended limit for 24/7 usage. Then I purchased renters insurance
I was looking at my power box yesterday at my shop which I am setting up at. I have a spare 70A 240V breaker (also a spare 100A 3-Phase). My shop has Industrial 3 phase I am guessing to be at least a 300A main line...given area averages I'm looking at less than .05/kwh.
So I think I'm good on power. Built a box which has a boxfan with a filter blowing in and a boxfan without a filter blowing out. Starting with 3 rigs ~3.1gh/s.
Now if I can only find more 5830's.
But on the question of 24/7...why wouldn't it be safe...just don't get greedy...no need to exceed the temp rating for that extra 1btc a month. Do the math on the cards/processor/mobo current use...add 10%(or more) and that is the psu you need. I think the main risk is drawing too much current, most modern PSU's can run on 240v also, if you wire into a 240v line you cut your current in half...be sure not to overload your breakers.