I only have a little grey in my beard so far, but like all optimizations it heavily depends on context. My broad rule of thumb is that if you only care what the code does, you should let the compiler figure it out. If you care how the compiler accomplishes that goal, you should specify that rather than hoping things don't silently break in the future. This is a fairly common thing in crypto and systems code.

But yes, fast inverse sqrt is obsolete.