The conventional wisdom is to use the lowest gain that delivers the volume you require. That will provide the lowest noise floor. In practice, it doesn't really matter for the majority of gear and people - except for those with golden ears that worry about everything. Set it where ever you need to for the volume you seek. If you have very sensitive 'phones, then you might need to use low gain to avoid hiss. If you have harder to drive 'phones, then hiss will be no problem and you might need high gain to get the volume you require.