I just usually make sure the surface is clean. If you want to use a
solvent, thats fine too. Any residual silicone that is not cleaned off
is usually not a problem. Just make sure you use high quality silicone
when you are ready to caulk. A tip, use masking tape in the corners and
edges so you can do a neat job without globbing any excess silicone on
the surrounding tile wall and bathtub.