Yes, no problem. Actually, I'd say it's pretty much the only "safe" way to get the Platinum trophy.
If you don't want to read why, well feel free, but it's because the game contains a feature called "World Tendency". There are five worlds in the game and the amount of souls you get from defeating enemies and other stuff can change depending on if a certain world is Pure White or Pure Black. To get a world a certain color, I suggest you read the Demon's Souls wiki (just Google it, it's the second link or something).
If you go back online, example one world is Pure Black, well there's a chance it will go back to being a Neutral world. There's a "server tendency" or something online and it depends on how many times other players die in a certain world. If players defeat more demons than die, well there's a better chance that the worlds will be white instead of black.
Anyways, some trophies require you to get some weapons upgraded to the max with certain stones. If you have a Pure Black world, there's a chance enemies will drop the stones more often. Also, you can still get your "Character Tendency" to black for the Mephistopheles quest by killing most of the NPCs which will net you the Foe's Ring, and the other way around applies for white Character Tendency by killing most Black Phantom NPCs in the different Pure Black worlds for the Friend's Ring.
Have fun!
