Now, from a server point of view there are two types of connections: inbound and outbound. Our servers accept connections but they also establish connections, for example to your http origin hosts.
So from the point of view of our server the "colliding" packets will fit two categories: A) incoming packets to port 53 B) incoming packets to outbound connections which source port % 32 == 21.
For A) this is not that a big deal. DNS usually works over UDP, there are not _that_ many DNS queries done using TCP.
For B), since Linux choses source port incrementally, that means every 32'nd connection will possibly have some packets hitting the unhappy bucket.
Therefore increasing the hash size twice, reduces the chance of collision twice: now every 64th outbound connection will have some packets hitting the unhappy bucket.
The full answer is: depending on which RFC you read :)
Initially the RFC's specified that you could only use TCP if you got UDP truncation _first_.
Nowadays that's relaxed but it's very vague when you should use TCP except for after UDP TR. For example Bind will try to connect over TCP if UDP fails.
Generally speaking most of the traffic goes over UDP, and sometimes, in undefined circumstances, some stuff may be requested over TCP. No hard rule.