I'm of the philosophy that code should be readable, reasonably self-documenting, and remind you of relevant external material - whether that be your state of mind at the time, or reference material you drew on, or whatever.
Not everything needs an open issue, but the knowledge should be recorded somewhere. I know for a fact I won't remember it in six months, and likely not in a week either.
(I also write a large amount of utility code that's below the "project" scale and will probably never see a PM tool, but which is nonetheless part of a heavy reuse ecosystem.)
