Yes, TARES is a Scrabble word!

Letter values in Scrabble US: T 1, A 1, R 1, E 1, S 1.

Is it a valid word?

TARES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

an adjustment made for the weight of the packaging in order to determine the net weight of the goods
any of several weedy vetches grown for forage
weedy annual grass often occurs in grainfields and other cultivated land; seeds sometimes considered poisonous
the weight of a motor vehicle, railroad car, or aircraft without its fuel or cargo
(chemical analysis) a counterweight used in chemical analysis; consists of an empty container that counterbalances the weight of the container holding chemicals

Also: bearded darnel, lolium temulentum.