I always use a clear coat before washes, or any other step that includes aggressive thinners. Not to say that the thinners WILL destroy a properly cured acrylic, but why run the risk? Same goes for acrylics, even though the thinner isn't aggressive, it COULD soften or dissolve the underlying paint.
Also, if you're applying decals you should always seal them under a clearcoat before doing any sort of enamel or oil wash.
World's Slowest Modeler