Welcome to the forum.
The "grainy" pictures are often a result of a standard-to-high-def converstion. Most Standard definition pictures will appear a little grainy on an HD panel, in your case a 1080p HD panel.
To directly answer your question, grainyness on one input or source signal will not cause the panel to "gain" any sort of grainyness over time.
The television, being a 1080p panel, will automatically upconvert any signal to 1080p, regardless if what is being sent is 480i/p, 720p or 1080i/p. But to reduce the grainyness, you want to send the best possible picture resolution TO the television. Most cable boxes, DVD players and Blu-Ray players have menus that allow you to toggle what resolution it sends out. For the least amount of grainy-ness, you'll want to send the best picture possible.
What might be happening with the Blu-Ray is that it's sending the picture quality out less than 1080i or 1080p. There are some instances where the stock film used to make the movie was less than ideal (one such transfer to Blu-Ray that comes to my mind is House of the Flying Daggers).
The best option, in the case of the Blu-Ray player, is to send a 1080p signal to the television. Then go to the setup menu in your other devices (DVD, Cable/Sat box, etc) and output the highest possible resolution. That should assist in reducing the grain.
Keep me posted.
Hi i own a ln52a650. I was curiuous if watchin to much standard tv and standard dvds can cause picture problems. My g/f watches her sex and the city dvds constantly for long periods of time. The PQ on those dvds are horrible full of heavy grain and whatnot, i know thats how they should look for the most part. But im curious now when i watch my bluray movies i notice alot of grain. I know some movies have grain but it seems i notice it more now than before. Can having the standard dvds playing to long cause bad PQ in the long term? any help with this is greatly aappreciated