No one has said that you can't do anything. You don't have to do it, but it's your story and you can do whatever the hell you like.
Now that I think about it, the authors of the books I read as a kid didn't take it upon themselves to italicize thoughts. For example, instead of they'd write The abuse of italics doesn't bother me, though, as long as it doesn't go overboard. Well, you italicized that for emphasis. Technically it's correct usage.
In my current work, I have used italics only for single word emphasis and in one other purpose: Look up. Christopher signaled to his team. I know that's probably incorrect usage, but I've seen it done in novels in the same genre as my own. Also, I've seen it done by several authors - so it can't be all that bad, right? I guess this would be better: Christopher signaled to his team look up. Or maybe even: Christopher signaled for his team to look up. However, I would like to point out that I italicized "look up" because it's a specific hand signal in the military. Then again, the average reader might interpret that differently.
There isn't a well-established standard for expressing signed communication in fiction. In textbooks, the individual signs (glosses) are rendered in small caps, with hyphens joining multiple-word glosses, but that really would be awkward for fiction. It also could not be adequately rendered in a standard manuscript format. It's probably most correct to treat it as ordinary speech in another language, rendered in English. In other words, regular quoted dialogue, with context indicating the mode of communication: I could see that this could become tedious if heavily mixed with verbal commands. The best compromise would be to punctuate signing as unspoken/internal dialogue, which is exactly the same as external dialogue but with the outermost level of quote marks omitted. In neither case should you italicize the signed dialogue, however. As a tag verb, signalled seems somewhat ambiguous. Until I reached the end of your post, I didn't realize Christopher was using hand signals. I pictured him speaking over a com link.
why not 'K.I.S.S.!' by just writing 'He signaled his team to look up.'? that makes it clear that it was a signal, not a thought or a spoken command... good writers never have to resort to fancy fontery or other tricks to get what they mean across to the readers... if you find yourself doing any of that, you probably need to improve your writing skills...
That works well for a single isolated instance. But in the type of fiction seta was talking about, there are likely to be complex series of commands issued in short order, all in hand signals. It is used almost like conversation, and there needs to be a way to convey it concisely.
in some instances I did find it was better to simply go with the: Christopher signaled for his team to look up. But sometimes I did interject italicized commands in the paragraph; Stop. Christopher heard something coming from around a corning. Look that way. Christopher motioned for Seamus to observe the flank... It wasn't exactly like what I have above, but the same sort of thing where it's almost a play-by-play. I don't use these types of things often (maybe 3 times in my novel so far) as my novel is people-focused, not combat-focused. I'll see if it still makes sense when I go back to proof read.
Sure, when they don't know how to make it clear to the reader that the character is thinking... Some, however, do it so well, they don't even need he/s thought tags, and definitely don't need to abuse italics.
Getting back to the whispering, How would you describe somebody whispering loudly? Supposing a look-out guy needs to tell the intruder (halfway in the window) that the owner of the mansion is about to turn the key in the front door. The look-out guy doesn't want to shout because he'll alert the owner but needs to whisper, or shout in a lulled voice to warn the intruder.
