Your fuel build up is based on your CFP and company policy. Your route knowledge and awareness of any differences between what is on the CFP and what you are likely to actually fly all add into the mix as to whether you would get airborne after a lengthy taxi delay. As would the length of flight.... A 16hr flight provides both the opportunity for fuel gains above CFP figures due airborne efficiency opportunities as well as a tech stop to top up the gas should you not gain any fuel along the way. A 1hr flight with potential destination ATC delays and/or potential weather related delays would potentially drive a decision to return to bay for more fuel.