The engineers recommend bolt replacement but try finding a new tensioner bolt in stock somewhere. Maybe at the Honda dealer? Most technicians will re-use the original bolt which should be fine as long as your not one of those guys that over torques everything.
I agree AND no ,at least at the dealership I'm at , they don't stock the bolt. Use a torque wrench- you might be suprised at how many bolts you're probably over-tightening. One example ,your spark plugs -try 13 FT. Lbs. , camshaft retainers on a 'Fit' engine 9Ft.Lbs. Lug nuts on your Civic-80Ft.Lbs. How many people do you see pushing on a lug wrench with their foot to tighten the nuts? Know what I mean?
The bolt does NOT need to be replaced...and whoever told you that it had to be replaced...ask him to replace the water pump bolts (wear and tear due to heat), camshaft bolts (constant vibration), valve cover bolts and grommets (to prevent leaks because you disturbed the valve cover gasket, crank bolt (high torque hardware), alternator, power steering pivot bolts (holding tension) and motor mount bolts (holds up torsion).
You might want to study -up a lttle more... The way many of the items you picked are assembled , when tightened to the recommended torque , they are under very little stress , although I can understand your perception of what they are doing. If a cam retainer bolt was subject to such severe 'vibrations' at 6000 RPM in a Honda Fit engine , you'd think they would use a bolt bigger than 6mm. in diameter and use more than 9 Ft. Lbs. of torque to hold it there. But I'm an idiot so what I say here is strictly the Opinion of an idiot. Have a nice day anyway!