While preparing roulades, set bowl of tartare into a large bowl of ice. This will keep salmon chilled for optimum flavor and freshness.
Combine first 7 ingredients in a metal bowl; cover and chill 30 minutes. Combine wasabi and horseradish in a separate bowl; set both mixtures aside.
Slice (1/4-inch-thick) ends from each cucumber, and discard. Thinly slice 18 (1/16-inch-thick) slices of cucumber lengthwise using a mandoline or vegetable peeler, avoiding center section of seeds.
Spread about 1/4 to 1/2 teaspoon wasabi mixture over each cucumber slice. Spread about 1/4 cup salmon mixture over wasabi mixture. Roll up slices, starting at short end. Cover and chill until ready to serve.
Arrange arugula on individual plates. Top with roulades. Garnish, if desired.