You wouldn't ever have back flow with a washer. Back flow occurs when you loose water pressure in the line, both of these lines would be pressurized by the main water, and would potentially back flow only if you shut off/lost pressure on main.

If anything you're more likely to get back flow from a fridge.

The fill valve is also well above any standing water (notice it flows from the top on washers). And even if that really was a concern you could just put a check valve in line with the washer hose.