Fill the weave. You want to fill as each coat is not tacky to the touch - for a good chemical bond. There's no advantage to waiting, plus you can get blush. Leave final sanding until the boat is complete with hatch and cockpit details done. You can always get some drips and scratches while working so there's no advantage to doing any final finishing prematurely. Sometimes I do some sanding if I'm bored but generally I do the entire sanding at the end because it's a dirty job.