If the wiki pages are held in a Wiki Structure, then you assign permissions to the structure, otherwise you have to assign permissions to each and every wiki page.
I use a "very locked" tiki 1.8 installation at damosoft.net for managing customer projects, i.e. each customer has a wiki area for documentation, trackers for bug reporting, forums which shows cvs commits via the forum mailin function. also another forum for almost staff to staff support and others.
Each forum, wiki area or tracker is only visible to certain Groups and all permissions are set on the objects themselves rather than globalling through Admin->Groups.