There is no such thing as "mf on" or "Mf off" it's all roll of the dice and based on numbers.
Ideal MF to run is around 300-350 (I trial-and-error around that range and do fine)
Download Atma and look at the probability calculator.
http://atma.diabloii.net/After downloading, you go to Utility -> Drop Calculator -> 1.11( it doesn't matter that it doesn't say 1.12 since all 1.12 did was make it so you don't play with a cd anyways)
This will tell you a lot..It is a very useful tool.
But remember, even though it tells you that higher the MF, odds of getting unique stuff are better right? wrong.. If your chance to get a COA at 500% MF is lets say 19,000 but its 19833 at 300% mf, you might wanna go with 300% because at 500% you are seriously increasing your chances to finding blues, and its all math and numbers, since you have way more odds getting blues, and sets, you'll probably get them more often
The trick is, balancing MF, and I've discovered that the balance is 300% in Hell Difficulty. But if you wanna find stuff like SoJ's, i'd say 600-650 in Nightmare Difficulty is your best way.
Hope this helps