Please don't.
The reason for using or not using frames should be based on a) your site's
needs, and b) your willingness to accept the potential problems that frames
can create for you as developer and maintainer of the site and for your
visitors as casual users of the site.
I am down on frames because I believe that they create many more problems
than they solve.
Judging from the posts here, and the kinds of problems that are described,
the kind of person most likely to elect to use frames is also the kind of
person most likely ill-prepared fo solve the ensuing problems when they
arise. If you feel a) that you understand the problems and b) that you are
prepared to handle them when they occur, and c) that you have a need to use
frames, then by all means use them.
As far as I know, the most comprehensive discussions of frames and their
potential problems can be found on these two links -
http://apptools.com/rants/framesevil.php
http://www.tjkdesign.com/resources/frames/
If you cannot understand these issues, then you should not be using frames.
Notwithstanding that rant, the percentage of browsers that don't support
them is zero, plus or minus a few tenths. 8)
