Is it OK to keep magazines loaded all the time?

[300rum]

This.

Imo. Leaving a mag loaded will not adversely affect a spring. All mine stay loaded. The cycling of a spring is what causes it to wear out or stretching or compressing beyond normal dimensions. Sitting loaded will not affect it any more than and probably actually less than loading and unloading frequently.
I learned about springs in engineering school.

[300rum]

In my main competition pistol I run about 20k rounds/year.  Cycles wear out springs, that's how it works.  I have mags that I practice with and mags that I shoot in competition.  At the end of the year the mag springs that I practice with (and cycle more often) are shorter then the less cycled springs.  In fact, by the end of the year the spring is about flush with the base pad when it started out about four inches below the basepad (they still run though).

Down loading will allow you to seat the mag easier, it won't save the spring.  Loading a Pmag to 28 will allow you to seat it easier.  Down loading a competition mag by a round will allow you to seat the mag with less effort. 

Speaking of Pmags, I use the dust cover to take the pressure off the feed lips so that the feed lips won't warp, that's what they are for.

Last, mag springs are a consumable item, keep lot's on hand and change them out whenever you have and issue. 

[BULLBLASTER]

  spot on with the consumable items. Similar to barrels. There's a lot of people that think they buy a gun and mag and are set for life. It takes maintenance and parts. Just like a car, oil change? New tires?
Granted 95% of people will never wear out a gun...