Quantization distortin right or worng?

Hm. Dither *prevents* quantization distortion at the cost of a low noise floor.
This should already be integrated in *every* hardware unit that converts analog to digital (ADC) and is and should be *not* optional. (Not aware of an ADC that had the option to turn this off.)
So, no, with modern gear you should not hear *distortion* building up with more sources converted from analog to digital. There is a noisefloor which is easier to handle and less obvious and that's it. Distortion could be the result of analog stages before the conversion depending on the quality of the converter.
Digital "coldness" like people still think of can be the result of other processing ITB like hard clipping, badly made software etc..
Much of the "you have to warm up with analog in the masterbus" is still from the 80's and 90's where many if not most DAWs still had flaws, people used wrong gainstaging with digital clipping etc.pp..
Nowadays you can mix completely in the box and also master ITB without a distortion that would be based just on the fact it was digital processing and not analog.
So, from a technical point of view, there is no reason to use an analog device in the masterbus to cover something up at all. Actually, strongly dependend on the quality of the converters, the extra AD-DA loop could do more harm than good if there is no reason for it at all.
Sure we might like doing analog stuff anywhere nowadays in hybrid setups, but not to cover up for quantization distortion.

Summing should be transparent in the box, it is addition in at least 32bit floating point numbers, which should not induce hearable distortion. Watch your gain structure ITB closely in plugin chains and take a closer look at plugins you use in tracks. It is not, that distortion can not happen in the digital domain. Unintentional clipping and noise surely can happen in the digital domain. Of course. But they shouldn't be result of quantization distortion or just the fact that you mix ITB, at least nowadays. Other things shurely can do harm. 10 or 15 yrs old plugins *can* have flaws, though...
After around 2000 sooner of later *all* DAWs improved their summing algorithms *alot*, nowadays they all use 32 or even 64 bit float internally and mix transparently if gain staging is right and they are not buggy...