A joint between wall tiles and a worktop without grout will not look very nice and will trap dirt/germs. Use tile spacers below the tiles to give a small gap for grouting once the adhesive is set. You could also add a silicone caulk after that to make the joint totally waterproof but it's a matter of personal choice and as it would be the last part of the job you will be able to see how it looks without it before deciding.
It's recommended to put some kind of caulking in between the tile and the countertop. It provides necessary support against gravity and prevents the countertop from becoming damaged in any way by the tile. It also makes it easier to get the tile out in the future when it's time for replacement.