I don't think they are optionally reflexive, it is just that some speakers omit the reflexive pronoun when there is little risk of ambiguity. For example, "lembrar" is better as remind and technically you should use "lembrar-se" for remember. I've found that Duolingo is often quite accepting of this practice (like Danmoller). On the other hand ..BATMAN.. advocates against it, particularly in a case like: https://www.duolingo.com/comment/8359387
Aproveitar is normally not reflexive. It uses a direct object like "enjoy something".
If you use the reflexive, you are somehow "enjoying yourself", not in the common sense it's used in Engligh, but rather like "you being the cause of your enjoying". Something really weird that I never heard in my life :p.
Of course there is the expression "aproveitar-se de". And that means "to take advantage of".
Dan, I don't follow "you being the cause of your enjoyment" changing the meaning of "enjoying yourself" in English, granted it's a cooked up phrase... So are you only saying aproveitar is not really a reflexive verb?
Aulete dictionary has this example: "É melhor aproveitar enquanto se é jovem." what is the se here? Thanks
So the "se" is pronome indifinido aqui, "It is better to enjoy while one is young", and the associated verb happens to be "ser" here... Makes sense to me now! Is the position of "se" in this structure always ahead of "é" (i.e "enaunto se é jovem" and not "enquanto é-se jovem")? Thanks again.