Unless you're writing code that could be useful on a DSP, you're completely entitled to assume bytes are 8 bits. All the world may not be a VAX (or an Intel), but all the world has to communicate, share data, establish common protocols, and so on. We live in the internet age built on protocols built on octets, and any C implementation where bytes are not octets is going to have a really hard time using those protocols.
It's also worth noting that both POSIX and Windows have (and mandate) 8-bit bytes. That covers 100% of interesting non-embedded machines, and these days a large portion of non-DSP embedded systems as well.