I regularly have to print images and posters on a large format HP printer (44" width X up to however long the roll is). Everthing that goes to that printer is optimized in photoshop at 150 PPI, then the image is brought into one of several programs. (Quark, PageMaker, Illustrator) These are to get the margins and exact placement correct. (as Slynky said Photoshop can get wonky) However, if you are outputting to a dye sublimation printer, then there is a visable difference between an image at 150 PPI, and the same image at 300 PPI. anything that goes to our Dye Sub is at 300 PPI. I am not sure how large you want to go, but if it is under 16" X 20", you might want to check and see if any color labs in your area are using and LED Print System. These take your digital file, and expose it on photographic paper that is processed in traditional chemistry. It is not cheap, but the prints I have had done this way are outrageous. They will want your file at 300 PPI also.