The filters UI doesn't really support this (yet), but you can specify the comparator when submitting the raw sieve filter via zmprov. After adding this filter, messages with uppercase "STUFF" in the subject get the "STUFF" tag, but messages with lowercase "stuff" don't:
Code:
[zimbra@support5 ~]$ zmprov modifyAccount f@test.test zimbraMailSieveScript 'require ["fileinto", "reject", "tag", "flag"];
# tag STUFF
if anyof (header :contains :comparator "i;octet" "subject" "STUFF" )
{
tag "STUFF";
stop;
}
'