I filter, eq, compress, saturate, etc always within the mix while sometimes soloing to check how it sounds by itself, but only very seldom adjusting while in solo. Mostly it's only If I have trouble hearing something in the mix.
If you're mixing your signals while soloing each channel you have absolutely no idea what you're doing because you're missing everything around it as your reference. Image eqing a signal, bass for example until it sound massive soloed and then switching back to the mix having it disappear completely, even though you filtered the signal nicely and boosted the low end a lot. But what you forgot is that EQs not only change the amount of boost or cut at a certain frequency but also change the phase of and around the affected frequency range depending on the design/type/algorithm of the EQ. You don't hear those changes in phase and what they do to your mix while soloing.
Same with compressing. Tweaking the attack and release parameters from within the mix helps you so much more to get the elements to sit nicely inside the mix. Do I want the attack of the signal to poke out a bit more of the mix or less? Can't really tell what's going on anyways when soloing... However a solo function with a dry/wet slider could be very useful for certain things, only slightly dimming all the other elements around the soloed signal.