With a digital scale it shouldn't matter at all, the sensor is an electronic strain gauge, increasing weight changes the voltage the sensor returns, the scale is calibrated to the change in voltage, translating that to the weight. There are no major mechanical parts to wear out. With this type of sensor having a light weight on it shouldn't make any difference to it long term. I have a measuring cup sitting on mine when it's in the drawer, and I've done that for years without an issue. A larger object should still be okay as long as it doesn't exceed the capacity of the scale, at some point it will damage the sensor. Although, if you are concerned you can just stand it up on its side instead of putting something on top.
Mechanical scales are a different story as most of them use springs, keeping a load on a spring long-term can lead to it deforming and/or weakening prematurely, leading to errors in precision.