I don't print my own decals very often , usually some license plates or a few logos . I use Testors paper only because my LHS has it and for about $6 I got 3 sheets of clear and 3 sheets of white paper . I don't see any problems with it , but if I were doing more involved decals ( like for a race car ) I might look at some better quality paper .
I also do the "cut and tape" trick for my decals , it saves me from wasting a lot of paper . As for decals printing transparent , I usually try to use the white paper and use MS Paint to match the background to the color it will be applied to .