I'm late to this discussion. Sorry if this is too little too late.
The washer on the crank sprocket doesn't sit flush. It should solidly engage the 'ledge', but it's thicker than the ledge is deep, so it will stand proud. That way it is pinched by the sprocket when everything is tightened down, and clamped securely in place.
The washer isn't flat... it's curved/ domed backwards such that it's outer perimeter leans back torward the engine. The rear face of the V-belt pulley is curved forward, and the washer is curved backward. Between the two, they form a funnel that guides the belt onto the crank sprocket. That is the feature that locates where the timing belt tracks, presuming all else is also correct/ straight.
If the washer is installed backwards, such that it curves forward over the sprocket's space, then it will push the timing belt forward. Once it's track is set 'forward', the belt will ride forward on all the sprockets. Flip the washer over so that it curves backwards toward the engine, then the belt should track noramlly.
All that presumes everything else is correct.
All the pivot axis are supposed to be parallel... crank, tensioner bearing, auxiliary shaft, and cam shafts. If something happens to cause one axis to be out of parallel, like the tensioner mounting stud is bent, then the axis will coverge toward some point. Whether they converge forward or backwards, the belt will track 'off the cone' toward the convergence point.
907s and early 910s used a spring-loaded, semi-automatic tensioner. It was articulated, and the 'hinge' could wear. When it got sloppy, the moving side of the tensioner would flop off-axis toward the most-worn side of the hinge. The bearing itself wasn't tapered, but it was running at an angle to the plane of the belt, and the belt would run off the 'taper'... forward or backward as chance would have it. Later spring loaded tensioners (all 910) have plastic bushings in the hinge pivots, and they should be replaced everytime the tensioner bearing is replaced. The bushings are cheap. If the tensioner is apart for any reason, replace the bushings.
One possibility that Travis mentioned is when a warped head it milled to flatten it. If an unknowing shop simply mills the bottom flat without also making certain the bottom is perfectly parallel to the cam bores, then the cams will be off-axis when the 'wedge head' is installed. That's a whole 'nother' topic, but it is absolutely NOT correct to simply mill a warped head !!
Everything about the tensioner bearing/ roller is straight and cylindrical. No tapers anywhere. And they're hardened steel, so it's very unlikely that they will wear to a taper.
The cam and auxiliary pulleys are also made without any tapers. They're aluminum and hard annodized with a dark olive drab to black color. If the outside perimeter (the teeth) are bright, shiny metal, then the annodizing has worn off. In that case, the raw aluminum will wear quickly, so replace the pulley. If a shiny pulley is wearing, then it's likely it will wear at something other than perfectly straight, and the belt will run off center in the direction of any taper.
Any pulley that has a raised edge around the front or rear is bad... replace it. It doesn't have a 'raised' edge, it's the center that has been worn down. If a pulley is worn enough to show edges, trash it.... whether the belt is tracking straight or not.