To set what? slime-net-coding-system? The default is already :iso-8859-1.
If a user is so enamored of Unicode, he should have enough clue about those coding issues and be able to read up about slime-net-coding-system in the manual.
It is not a matter of being enamored or not, but rather of offering a default that works whatever language the user works with, and in some cases "language_s_".
Jean-Christophe Helary