As a general rule, we do not recommend leaving wood pellets in your hopper. They are formed from compressed sawdust. If these pellets were to get wet, they will absorb a lot of moisture and completely disintegrate. This makes them completely useless for your grill. As they absorb water they will also expand, which can lead to them completely blocking up your auger. This will require a lot of time and effort to reverse.
During drier times of the year, where the rainfall and humidity are low, there is much less of a risk. They are unlikely to rot or disintegrate and will probably be fine for a short period of time. That being said, the official advice from all pellet grill manufacturers is to remove any unused pellets in between uses. It does not take much time or hassle to do.