A painted surface will never be better than the underlying surface. Filling and sanding can be laborious, but it is much easier and more enjoyable to paint when the surface has been properly prepared. But let's not exaggerate. Think of the final result, and fill accordingly. For example, you can sand down wallpaper seams first and then fill if required.
You can also save filling by cutting small wallpaper bubbles, lift the flaps and apply new glue – followed by a thin layer of filler.