Regarding other threads:
It pretty much depends on the individual thread if it wants to wait for the network to come up. I dont think someone else than the thread itself can decide if it has to wait for the network. I would say it is in the responsibility of each thread to handle not-upped network interfaces themself.

blocking the GUI unconditionally:
I would suggest to disable the whole wait-for-net bevhavior by default and let the user enable it by an advanced setting if needed. I dont think its good to just trigger it my something like "IsUsingMysqlDb()" because there might be also other situations where you would want to wait for the network.