I've seen the usage of English phrase for what it's worth many times.
However, I still don't know what the most precise translation of this phrase is in Russian.
No direct translation exists. 'For what it's worth' usually means that whatever is to follow is going to be a subjective opinion, which translates well to
If you want to communicate that a piece of information is potentially unimportant for another reason, you could say something like 'Может, это неважно, но'.
FWIW is roughly synonymous with
my two cents, which can in turn be translated as
мои пять копеек
UPDATE: Note that
мои пять копеек is highly colloquial and can only really be met online.
It depends on the context. If you just add some information which value you are not sure about, I would translate it as
на всякий случай. If you express your opinion, you can add something like
я не настаиваю, но or simply
Не знаю, на сколько это важно/полезно, но...
Может быть, это и не важно, но...
"За что купил, за то и продаю"?