Subversion tagging plugin donesn't have that feature. However you can achieve this in the following way.
No need to tag via subversion tagging plugin.
Use some script like the following which will tag the sources.
rel=yes
svn_tag_path=http://mysvn.com/svn/repo/tag/
if [ $rel = yes ];then
echo "Tagging the release Candidate";
svn copy $WORKSPACE $svn_tag_path/$JOB_NAME_$BUILD_ID -m "Tagging the release candidate from revision $SVN_REVISION"
fi
set the value of the variable rel
in the script to no
or something
for normal builds. For release candidates make the value as yes
.