My take on this type of operation is for W&B purposes, plan to use flt fuel + taxi fuel for the leg. Should you use any variable reserve then your LW is conservative and your next TOW is also conservative unless you refuel. Always ensure that the a/c will be in trim from your T/O fuel load to empty tanks with no exceptions.
For FUEL planning on a multi sector day, plan to burn flt fuel, taxi fuel and variable reserve each leg so that your planned fuel for the next sector is conservative. Is is a legitimate method to plan for variable reserve for the longest leg only with the caveat that you may run into a problem should you burn some of your variable on the early sector(s). A matter for your judgment.