Drain hole is above bottom of water heater so about a quart of water can be left when drained.
This amount of water will not cause an issue cause there is lots of open space inside tank for water to swell when it freezes.
Drain it/flush t to get crud out/clean threads up/install drain plug (so threads are protected and critters don't crawl inside set up house)
IF it really bothers/causes you worry..wet/dry shop vac with small necked tube that can go in thru drain hole. Suck remaining water out......BUT not necessary
