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.
I use MTS/Sand cap my initial plantings are done with damp (not soaked) substrate. I find this to be the cleanest method for dirtied tanks. I plant crypts/swods deep then the day after filling the tank I gently pull them up to get the crowns a bit above the substrate. By waiting the MST/Sand has had time to fully settle.
If you do it with the tank full, make sure you leave enough headspace for sloshing of water and for your arms to go in the tank; more of an issue on smaller tanks. Not fun going in to work on a plant and realizing that your hands and forearms displace enough water to overflow the tank.