Is a 'least significant bit' used anywhere practically today?

I'm working on a library that supports file reading, and right now I've always assumed that all the bytes are written in most significant bit first format. Is the least significant bit first format ever practically used anywhere today on any platform? The library I'm writing isn't targeted at older operating systems (ideally anything onwards from the year 2000).

I'm curious if writing such a feature in might be a waste of time that could be spent on other things.