I don't think leaving magazines loaded affects springs at all. Springs wear out from being compressed and uncompressed over and over again. Either leaving them compressed or uncompressed shouldn't affect them.
HOWEVER
I don't usually leave my magazines loaded because of the legalities involved with transporting my weapons back and forth from the range. I don't have a CCW permit. Generally, you are required to transport firearms unloaded, in the trunk (or inaccessible), etc. But, I drive a truck and don't want to put the guns in the back bed. So I leave my guns in the back cabin. I keep the magazines unloaded and the guns in separate compartments from the ammunition. That is probably good enough for my locale and interstate travel.