The trucks on model steamers do nothing but represent trucks. It's how they are engineered, assembled, and then how they ride on buddy's rail construction that determines whether or not there will be problems.
Some trucks are designed to be forced down, lightly, to ride better. They have a leaf spring or a coiled spring between them and their mounting post for that purpose.
On real steamers, trucks are there to support actual weight and to help steer the locomotive into curves at high speeds (this is the main purpose of the pony or engine truck just aft of the pilot). There are no centering devices on our model trucks, and they do not support weight...that is done strictly by the drivers.
However, one soon learns never to say never and other categoricals, both in the real world of trains and in our models (anyone remember MTH's HO scale Union Pacific 9000 series 4-12-2 with it's articulated front end?). Some higher end steamers have sprung drivers, and it the weight distribution is weird, it is entirely possible that the rearmost or foremost driver axle gets compressed a bit. If that happens, it is obvious that the frame will tilt forward or aft, and if forward, the pilot's lower lip may scrub along the rail heads, especially when vertical curves upward are encountered.