Do you use decal solutions?

Micro System for most.

Sometimes Solvaset.

Micro Sol and set. I have never tried any other. Solvaset is not avliable in the UK and i was only recently aware of other brands.

Gunze's Mr Mark Setter (blue bottle) can be a lifesaver when working with decals which don't have enough glue (whether badly made or over soaked). It contains a mild adhesive to help the decals stick. It's actually white, but dries clear.

Micro set and sol when I have to use a decal.

Some people seem to believe setting solutions and solvents are the same thing.  I usually only use setting solution unless the decal will not bend and form properly in highly curved surfaces.  And then I use solvent very sparingly.  If you are not used to using decal solvents, I would practice on spare/unwanted decals before using it on good models.

I have found some brands of decals that deform and stretch even in setting solution, so I don't use it for those brands.

I find microscale products the easiest to find at my LHS.

As little as possible, and I start with Set. I try to avoid Sol.

Tamiya has a decal setting solution called "Mark Fit."  It comes in the same kind of bottle as their thin cement, only with a yellow cap.  It seems to be very mild, with little chance of causing any damage to decal or paint.  I have used it and it works fine, although maybe a little slower than others.  I would call it a setting solution, rather than a decal solvent.