Vox that don't need de-essing?

I only use de-essing when the vox are stronly compressed, cause sssshhh become weird.

Training? Well, probably in a small precenteage, I think mouths are different and some people accentuate more the "s". So it is a matter of how do you pronounce them.
 
About SSL de essing:D:

From Manny Marroquin SOS interview:

“I also used an SSL side-chain to de-ess the vocals. The 902 is a great de-esser, but it only takes away one
frequency. On the board I have a full spectrum, and I use the side-chain to grab other frequencies. I have more
control by using both the SSL and the Dbx. What I do on the SSL is an old trick I learned from engineer and mixer
Barney Perkins, who used to do all those Babyface records, and it makes the vocal sound. There’s no de-esser
that sounds better than this, but it’s a technique that’s complex to explain.
“Basically, I route the vocal signal to two separate channels on the SSL, say channel one and channel two, which
are right next to each other. Channel one is my side-chain and works like a de-esser on channel two, which
becomes my actual vocal channel. On channel one I’ll set the SSL compressor to a fast attack and also engage a
high-pass filter — ie. I filter out all the low end — and do extreme EQ’ing of whatever frequencies I want to take
away. I don’t cut, I boost these frequencies; +12dB with a very narrow bandwidth, most often around 6-7k, where
most of the ‘esses’ happen. So channel one accentuates what I’m trying to take away! Of course, I take channel
one off the stereo bus, so you don’t hear it in the mix.
“I then press the Link button to link channel one to channel two on the right. I engage the compressor on channel
two, and what happens is that the more I’m bringing up channel one, the more the compressor ducks the
frequencies I don’t want on channel two. So my channel one fader becomes my threshold. I take out the bass
frequencies on channel one, because I don’t want them to disappear in channel two. With side-chaining, the
frequencies you’re accentuating in your side-chain are ducked in the other channel. I’ve been part of forums during
which I’ve discussed this technique, and people found it very hard to understand. Even the people at SSL have a
hard time understanding why it works! But it does. The key is that you can use compressors either to increase gain
or to create a ceiling — ie. they also work like limiters. And if you were to use straight EQ’ing to take away your
‘esses’, you take all the life and presence out of the vocal. But if you de-ess this way, it retains the personality of
the singer; it’s like he or she is in the room with you.”
 
My only qualm with that technique is that the SSL compressor is a standard, broadband compressor, not multiband. So as soon as those 'esses' come out, the compressor ducks the entire signal, whereas a de-esser or a multiband comp would just duck that one frequency range. Doesn't matter what you're boosting in the side chain signal, the compressor acting on it is still a simple broadband gain reduction unit. It seems like it would've been a good technique back in the old days of mixing on an analogue board, but I think these days of awesome plug-ins give us much simpler solutions!
 
Sometimes it's also an issue of how the mouth is build. some people can't or can avoid doing noises while speaking some not.
My "S" sounds very sharp while speaking, but while singing it's not that obvious...

The sharpness of your S and T-sounds are directly related to how wide the gap between your front teeth is.
K-sounds varies depending on the width and depth of your palette.
Your D's are determined by both your front teeth and your palette.
P's and B's are determined by how hard and thick your lips are.