I usually only use it when there is a real problem with ISP routing such as when undersea cable having issue. During such time the routing is very congested in the normal route but WTFast can route your connection via other country which is lightly used and hence improve your ping. During normal usage, WTFast normally can find slightly better route for you, but I don't find it significant enough to really care about it.

You won't get banned for using it because it merely changes how your network traffic reaches the server.

Usually I just use auto routing. I only use manual settings when the auto routing does not produce good result. Traffic info is dynamic so there is no fixed server one can rely on the get best result, just a general guide. Usually the Singapore server is the one that produce best result (around 100ms ping) to oceanic servers. It is usually either SG7 or SG8.