That is your choice as I have done it both ways and they both work. Don't cover the foliage any more than an inch however as they need sunlight to produce the tubers and it will take the greens a few days to sprout from the soil covering. If you have the time and self discipline adding just a bit of soil every other day while keeping some green above the soil line is easiest on the plants. Congrats on your potato barrel growing project. Soon you'll be singing "this spuds for you"  as you enjoy your home grown taters. Keep growing, Marianne Binetti