The only time I plant empty is if I'm doing a dry start method for carpet plants or moss. Otherwise, I plant at half to 3/4 full. Benefits:
-immediately lets me know if plant is secured, floaters are obvious
-lets me see how the final planting is looking if stems/leaves are upright rather than drooped over
-taller specimens aren't constantly in my way as the water bouys them up so I can reach under and around them.
-easier to fill the rest of the way without disturbing the work I've just done.
-no worry about drying out if I decide to take a break or sit and stare at the tank for half an hour trying to figure out a problem.