it shouldn't have noise. the printers don't mind it if it's a little soft up close. they don't like blocks, stretching, jaggies, and noise. if you have a digital image, and you enlarged pieces of it, they might not print it because its not even. noise is the same. if your using a good scan of a painting it shouldn't matter much. i use composite, and have to be careful of using textures that have blocks up close, or things with noise and even it out if it has any.
i will make a new layer using a median of 15. this will blur the heck out of the picture, but still leave contrast edges (don't use Gaussian blur which blurs everything). set that layer mask to black so it vanishes.
then use a soft brush set to white, and on the mask use a tablet to pen in the areas that are noisy. this will help converge the noise, but you have to be careful not to mash detail too much. i use this on all my images that needed. skies, walls etc
if you use blur and you get near an edge it create a sort of sprite near that edge and it will look bad, like its glowing. median will prevent that. median is also good for rounding masks off so you have nice smooth lines after. its how i cut everything out. quick dirty lasso on mask, apply, select the object make a new mask and run median on 30. and it rounds the corners, but it also remove thin things.
---Mike Savad
MikeSavad.com