My experience with this on a V8 Toyota 3UZ-FE, was I replaced an idler and then a wheel off of the tensioner. The tensioner cannot come off the engine without more work, so I bought a new tensioner and took its wheel off (ended up being same price believe it or not as getting the wheel separately).
yes, when I replaced two wheels, the squeal went away. But I learned from the above, it was in fact the smaller idler that needed the replacement, not the larger wheel on the tensioner. I guess when DIY it's not the end of the world when we replace something unnecessarily esp. if < $100, but just throwing it in there, seems to be smaller wheels/bearings that go bad as they are spinning more...