I have heard it can be bad to fill up your magazines completely and then leave them like that because it reduces spring tension but what about if you leave a drum full will it reduce the tension of this also?
Springs lose their tension as a result of being compressed and released. You won't damage springs by leaving them compressed, as long as you don't compress them past their design limit. If the springs rust, that's another situation. I know of some inherited .45 magazines that belonged to a friends' uncle. (WWII vintage) The outside of the mags were badly rusted, but the rounds popped out, and the follower did its think just like it was supposed to. Best I know those mags are still being used.
It's fact that the BEST way to insure longer life of mag springs is to have 2-3 times amount of mags you intend to keep loaded and only keep 1/2-1/3 of the mags loaded and rotate them. It's also a good idea to keep 2 less than maximum capacity. Everybody kept their m16s loaded in this manner of 2 less than maximum but we didn't worry about it with our M40s.