Ah. I was reading code not documentation. :)

SV *
Perl_newSV(pTHX_ const STRLEN len)
{
    dVAR;
    register SV *sv;

    new_SV(sv);
    if (len) {
        sv_upgrade(sv, SVt_PV);
        SvGROW(sv, len + 1);
    }
    return sv;
}
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I believe I::C will take care of that for me.

Indeed. Inline::C-Cookbook makes that clear (though doesn't take the credit):

In this example the sv_2mortal call gets done under the hood by XS, because we declared the return type to be SV*.

Thanks.