Three schools of thought. 1. Don't fill the filter at all and bleed the whole thing through. 2. Have a cup of fresh fuel handy - undoubtedly the best option, but unless you have a simple way of extracting or storing fuel, entirely impractical. 3. Use the old fuel. If it's a routine service and you have nothing wrong with your fuel system the 'crap and dirt' would be minimal... But it would also be trapped by a brand new clean filter.
