Not bein' a wiseass here, but with or without idler, if a belt stretches, your timing changes . . So the best bet, every year, is to put a new belt on. Especially if you're running a lot of open valve spring pressure and short ratio rockers . .
On our car, we have no idea of how old the Jesel drive is, could be 5 years, could be ten, dunno, it was on the engine when Doug bought the car, but we recently had to replace the gear with the guide plate on it because the spot welds gave out and all that was keeping the belt on was the pointer . .
But, for the last three years of competition, we've changed the belt every year and had no problems with timing or other timing related problems . .
Just my nickel's worth . .