Could i ask why ? most of the times even B string is not usable when open...
Depends which style of which is played.
Traditionally, the low B isn't always used. But, that doesn't mean a low F (lower than B) is never applicable.
I wouldn't go any lower than an E an octave below a 4-string (as you can actually clap the note, it's that low).
In many newer styles of Metal, with the popularisation of the 8-string guitar has lead to bassists with 5-strings just tuning the E A D G strings up a whole tone (F# B E A), to match the guitars. Problem is, it's in the same octave.
The role of the bassist becomes overshadowed and serves only a sound, not a bass role relative to the guitars.
I've been listening to some Prog Metal that uses 8-strings and know that if I were to use a tuning that's an octave lower than the 8-strings (as some bands do) my 4-string can pack a real punch in the mix.
If I'm to play any other style, I would use my 6-string though.
EDIT: Also, there's not much point adding a lot of distortion onto the bass, if it's tuned that low it's meant to sound huge and rumbly. Without clean signal, a lot of those lows can be lost.
I'm going for impact for the song, not just for low notes because I feel like it or something.