Why does this particular question never die...it always comes back like, I dunno, it's spring-loaded or something.
Anyways, answer is that leaving your spring in its current state, be that compressed or uncompressed, will have no wear effect on it. The cycling action is what wears your spring out. Those who load and unload a magazine every night to keep the spring from wearing out are doing exactly the wrong thing.
That said, if you leave a magazine lying around for an extended period of time, it'll likely get dirt, dust, and other crud in it, which will cause feeding problems.
In your particular situation, as long as the magazine you're leaving loaded is getting used (and hopefully also cleaned) regularly, you should be fine. If any of the more experienced members or those with Glocks can confirm or correct that, please do.