The difference between tabbed washers and flatted axles alone isn't much, but it's a large relative change from one to the other. The bare axle in a snug fitting slot applies torque at two points, both at a radius of about 5mm. A tabbed washer moves one of these points out to a radius of 12 or 14mm. It's a noteworthy change that makes frame failure less likely. I'd do it because there's not really a downside to it.