The larger the radius the better, of course, an o-54 loco will still run smoother through an o-72 than an o-54. Lionel/MTH engines are rated either o-54 or o-72 because those are the radius progressions they offered for a number of years. Theres always a chance a loco might run on a radius tighter than its rated for, but I've found even if it does it may look like crap doing it. When the center two drivers of a scale berkshire are hanging off the inside of the turn away from the outside rail it just looks silly.
"Generally speaking" as far as Lionel/ MTH are concerned,most 8 wheeled locos i.e. berkshires and mountains are rated o-54 with the exception of larger northerns like the UP 844 with its centipede tender.
10 or 12 wheeled rigid frame locos are all o-72.
Most articulateds are o-72, with the exception of Lionels recent legacy usra mallets which, being early articulateds are rather small.
Diesels are more forgiving but they can look silly when the fuel tank is hovering to the inside of a turn or clips the switch stand going through a turnout.
The majority of brass manufacturers locos articultated or not, require o-72, with their more prototypical detailing and close coupling.
I would say stay with the manufacturers recommendation, unless you can test or know someone who has run a particular loco on a tighter than recommended radius.
