if you're just autoXing on them, a 225 on a 6" wheel is fine. it'll stick out and look funny, but plenty of people have run 225s even on 5" wheels with success. if i was running my toyota in HS, i'd be running 225/45/14s on my stock 5.5" wheels. the tradeoff is in transitional response, and in the case of a REALLY tall 225/50/16 on a P5, acceleration.
i don't think a blanket statement like "no 45 series or lower tire should be mounted on a 6" rim" is entirely accurate. that is going to depend on the width of the tire relative to the rim; and for the record, a 205/45/16 fits perfectly well. staying within the manuf's recommended range is never a bad idea (and for street use, it's a very good idea); however, keep in mind that ass-covering (and product performance) is a major concern of tire companies. the car isn't going to burst into flames on course b/c a wheel is 1/2" outside the recommended range, but the tire might not be performing optimally if the wheel isn't the proper width.
as a practical matter, i probably wouldn't run anything wider than a 215 on the stock 6" wheels...the tradeoff in response probably would not be worth it. i believe hoosier or kumho makes a 215/40/16. on the street, i wouldn't bother going wider than a 205.