I don't have specific experience with Toyota Tensioner and idler but on the GM ones that I have worked on I've been told to check the bearing on the idler i.e. check for smooth running and no wobble/play. On the the tensioner there should be sufficient tension and no side ways play which would cause the idler to be angled to the other pullys.
If all checks out why change? But like anything that looks good today may crap out tomorrow.

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