```
$ cd net/instaweb
$ git grep 'const StringPiece& [a-zA-Z0-9_]*[,\)]' | wc -l
1271
$ git grep 'StringPiece [a-zA-Z0-9_]*[,\)]' | wc -l
420
```

I'm inclined to do whatever's the most common unless there's good reason otherwise.