Irving, a spring will in effect change the SWR of the antenna, but it will prevent damage if you accidentally drive into a garage, parking garage or tree limb. It will reduce stress on the mount.

Ideally an antenna should be as long as possible, mounted as high as possible and centered on the ground plane to get the best transmission in all directions. Barring this, everything is a compromise (what's new?)

The SWR will let you know if you have a good ground as well as allow you to "center" on the frequencies (channels) you want to use. Example: if you only use the lower half of the channels including emergency 9, then you may want to make them the optimum. But if you want to average over all the 40 channels (assuming the antenna can do it), then center it on 20, just make sure it is still good enough to use channel 9.