UDP always precedes TCP. TCP can only be tried if UDP fails or if UDP was disabled at all.
For making DNS MX queries, leave this value true (some DNS servers like Google's 220.127.116.11 provide meaningful replies in TCP only). The same applies to DKIM/DomainKeys checks.
For making DNS RBL queries (actually, DNS A requests), it's better to disablefor performance sake. The nature of DNS RBL query is that if no requested A record is found, it simply means the respective IP address is not in the blacklist, it's not an error of any kind. Therefore retrying with TCP would be waste of resources.
If DNS UDP queries are disabled (UdpRetryCount is zero), this property must remain true (otherwise, neither TCP not UDP would be available for making a DNS query).