Here's the Law on it. So you can roll the dice all you want. It is illegal.
Fenders or splash aprons.
(1) Except as authorized under subsection (2) of this section, no person may operate any motor vehicle, trailer, or semitrailer that is not equipped with fenders, covers, flaps, or splash aprons adequate for minimizing the spray or splash of water or mud from the roadway to the rear of the vehicle. All such devices shall be as wide as the tires behind which they are mounted and extend downward at least to the center of the axle.
(2) A motor vehicle that is not less than forty years old or a street rod vehicle that is owned and operated primarily as a collector's item need not be equipped with fenders when the vehicle is used and driven during fair weather on well-maintained, hard-surfaced roads.
[1999 c 58 § 2; 1988 c 15 § 2; 1977 ex.s. c 355 § 41; 1961 c 12 § 46.37.500
. Prior: 1947 c 200 § 3, part; 1937 c 189 § 44, part; Rem. Supp. 1947 § 6360-44, part. Formerly RCW 46.36.130