The master bedroom of my house had painted wallpaper when we bought it. I hated the look of the seams and the paint had gotten a crackle texture, presumably from the expansion/contraction of the paper under it. I worked for nearly a week, using DIF and a "Paper Tiger" (a little gadget designed to be used in conjunction with DIF to remove wallpaper) to remove the painted wallpaper, and swore at the previous home owner the entire time for having done this (to me?!). I think the old adage "an ounce of prevention is worth a pound of cure" is relevent here, I recommend removing the wallpaper before painting, or you will be doing many times the work later. Good luck!