In an interactive shell,
$IFS will be the default unless you changed it or something in shell startup scripts (like
bash) or any other scripts sourced or shell functions executed since the shell was started change it (they generally wouldn't or would change it back to the default if they are properly written though).
If those functions defined in rc files or other sourced files are properly written and they need to use word splitting, they would not assume anything on the current value of IFS, so it should be fine for you do set it to a different value.
However, once you change it, you have to remember that you did as that might affect the way command lines you run later are interpreted.
For instance, if you do:
IFS=:; ls -- $PATH
To list the directories in
$PATH, you might wonder why later on things like:
rm -f -- $(cat ~/my.file.list)
do not work any more. So you might as well write it:
(IFS=:; ls -- $PATH)
IFS=:; ls -- $PATH; unset IFS
unset IFS restores the word splitting to its default behaviour, but not
$IFS and it breaks the (not properly written) functions that do things like
oldIFS=$IFS; IFS=xxx; ...; IFS=$oldIFS to restore