Rectangular wings can get away without washout because that planform tends to have favourable root to tip stall progression as a characteristic of the rectangular shape.
The stall strips aren't really to change the root to tip stall behaviour as a substitute for washout; their job is to generate a local flow separation, just prior to the natural stall, who's turbulent flow can be felt through the airframe and at the elevator while the rest of the wing is still unstalled. That's why they are just short sections in front of the outer parts of the horizontal tail, so that the burble they produce will be felt through the controls well before the rest of the wing lets go.
The Yankee had a NACA 64-416 laminar airfoil which had an abrupt stall and very little natural pre-stall buffet - helped by the strips. Later variants got a revised airfoil with a drooped nose to make the stall itself more gentle, but they still retained the strips to provide the pre-stall buffet.
Many years ago I flew a Yankee that was in a flying club I was in, and it was considered a demanding airplane. The stall strips made it a bit less dangerous than without, but it still took careful speed control when landing. If you got too slow it would start to sink and pulling a bit more pitch had no effect. It had a reputation as a very "hot" airplane for something with only 108 HP.
On a tapered wing, you would be adding stall strips for the same reason as the Yankee's wing, near the root forward of the outer end of the horizontal tail, to provide a distinct pre-stall warning buffet. If your tapered wing had no washout but had the strips, all that would happen is you would get a pre-stall buffet that wouldn't be there without them, but then when you get to the wing's stalling AOA the whole thing goes at once because it's still a tapered wing with no washout.
Some designers get some of the benefit of washout without twisting the wing by varying the airfoil section itself from root to tip.