I don't think there are many tuners who don't silence the ranks not being tuned, but I have heard of some. As Classic Car Man said, it is easier in the bass and more difficult in the treble. If a mixture is not too badly out of tune, I will do as much as I can without stopping off other ranks, but it is important that at some point in the process you have all ranks going and what you can't (or certainly shouldn't) do is only tune the ranks individually. Doubled ranks do make for problems occasionally and even where ranks are not doubled in the mixture itself, one does get instances where ranks such as the 2ft will fight between the mixture and the Fifteenth, for example.
What is also important and something I do without fail, is to tune mixtures together with the rest of the chorus going as well. That does make tuning the mixture more difficult, but it ensures that the mixture is in tune as it is used, and not simply on its own.