Well, it's hard to say, really. It depends on what you're doing. The benefit of converting to UTF-8 when making a string from bytes is that string operations have predictable performance, and strings have predictable memory-usage. But of course, you then have to pay the cost of converting to UTF-8.
On the other hand, if you just track the encoding in your string type, then you don't have to pay a conversion cost at the boundary, but each encoding will have different memory-usage and performance characteristics.
