I've been using reducer rings for some years and provided there're of good quality (Freud, Trend etc) no problem. Just best to ensure that the blade and machine are compatible from a safety point of view.
I have had problems with a wicks circular saw which has a 20mm bore when i tried to fix a 30mm bore blade. The problem was not really the reducer but the locking washer which wasn't quite big enough to grip the blade being designed for the smaller 20mm bore. Worth looking out for if you are using a bigger blade
I've had a similar problem to bilzee with a Skil saw in the past too. As long as the locking washer still grips the blade and not the reducing ring it's not a problem, but certainly something to be aware of.