I have played that trick of bending a sideframe a little to make a wheelset roll better. My experience is that sooner or later, it will always return to its original position. It may not happen today, maybe not tomorrow, but soon and for the rest of its life (apologies to Humphrey Bogart).
You need to permanently deform it, not just bend it. Find a spot on the inside of the sideframe where it won't show, make a small "scar" across the plastic with a soldering iron and bend the bit of sideframe so that it bends at that scarred joint while it is still hot. This permanently reshapes the plastic, rather than just bending it, which is essentially just stretching it. This also reduces the risk of snapping the plastic, because it will give at the weakest point, where your scar is. This always seems to work much better for me. It's easier if you clamp the think in a small vise while you do this, so you can hit it with the soldering iron and pull on the part at the same time, then hold it while the plastic cools so it stays in its new position.
