I'd say use fx channels Only when you want them as "last in the chain" and busses when you want, or might want to bus/send them further on, or if you might use them as sidechains for stuff on other channels.
I pretty much only use fx channels for some of my delays and reverbs. Busses can be used the same as FX channels though, but I think it Might make for lower latency in songs, if fx sends that go last in the chain are kept on FX channels, they will be handled last by the computer. Just my personal guess.