I don't know what DNS uses the randomness for, but if a malicious attacker can gain from guessing the randomness, don't use MT, as the state can be extracted from MT by observing a relatively small number of outputs.
I believe you would use it to determine a random outgoing port to use to contact the DNS server; this prevents spoofing. However, the port space is only 16-bits, so how you map the outputs of the MT into that space would have the biggest impact -- but you're right that's it's probably best to avoid it entirely.
You also use it for randomising the 16 bit ID in the request packet, and you can use it for randomly capitalizing letters in the qname (0x20 bit encoding). Both of these to help protect against spoofing.
[edit] I'm going to skip using the Mersenne twister engine and just use std::random_device for all random data, instead of as a seed. It seems on Linux at least that random_device is basically /dev/urandom. I assume the source will be sane on other OS's too.
The C++ standard gives little guarantees for the quality of std::random_device. As I remember, MinGW on Windows uses the Mersenne Twister with a hardcoded seed for it.
boost::random_device, on the other hand, has better guarantees: it is only implemented where there's a decent entropy source.
This is a recommendation that says that a developer should actually consider using the Mersenne Twister for secure random numbers to avoid a potential performance problem.
