Question: I have a Canon 700d with 2 kit lenses, both having auto-focus feature. I read a book "Canon EOS Rebel T5i/700D For Dummies", which warns against switching auto-focus on/off while the camera is on. According to it, we should first turn off the camera before turning auto-focus on/off. That's quite cumbersome obviously.

A few days ago, a friend of mine who is professional photographer noticed me doing this, and told me that doesn't matter and I need not turn off my camera while switching auto-focus on/off. I am confused if this might harm my camera or lens in any way?

Answer: 
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> According to it, we should first turn off the camera before turning auto-focus on/off. That's quite cumbersome obviously.
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That seems pretty ridiculous. Perhaps the authors are serious about their target audience being dummies.

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> I am confused if this might harm my camera or lens in any way?
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**You won't hurt anything.** It's fine to turn AF on or off while the camera is turned on.

For evidence, see page 170 of the [T5i manual (PDF)](https://static.bhphotovideo.com/lit_files/79893.pdf), which covers manual focus operation. Note that it instructs you to switch the lens to `MF` but doesn't mention turning off the camera first.

As Rob points out in another answer, you should probably avoid changing the focus mode switch while the lens is actually autofocusing, as it would be if you keep the shutter release half-pressed or set the camera to Continuous AF mode for shooting video. So don't do that. Changing the focussing mode or other settings *during* use seems like a strange and unlikely thing to do, though, and it seems unlikely that you'd find yourself tempted to do it.
The warning you read applies to Live View shooting. If you look at the T5I manual it states on page 156 under the Continuous AF section , "...During Continuous AF, turn off the power before you set the len's focus mode switch to MF". I usually only use Continuous AF on my T4I so if I do switch to manual I don't usually remember to do this, and I have never had an issue. With that stated, following the camera manufacturer's advice is the safe way to go.