This is an optimization that should be done only when clear proof exists that the one with the strings is too slow.
I'd add that you should go ahead and set up a way to change which method you use. Do a typedef or define something like ASSETSTRING to string. When trying out hashed strings all you have to do is change the define to your hashstring class, and everything will automagically work. If you just stick regular strings in your code, it will be virtually impossible to find all the right strings to replace, and you will probably introduce a lot of bugs. Later, when you've settled on an implementation, you can find+replace ASSETSTRING to whatever if you want.