is it better to do this in the fridge or just at room temperature? I have made a batch that I will be using tomorrow. Also, when I come to roll out is cornstarch or powdered sugar better to use? Thanks!
I store my MMF at room temperature but I know that there are different ways probably depending on the recipe.
For rolling out I use cornstarch but for my last cake I used shortening for modeling and I think this works better. I've never used it for rolling out. With the shortening you don't have a dusty surface and it doesn't get dry.
Luggi
If I use shortening, won't it have a greasy sheen and taste odd? Also, isn't there more chance of my borders, sidework etc just sliding off the greasy cake? Help, I'm confused! 
You don't need very much shortening! As mvucic wrote "a tiny bit" 
My suggestion: Try it with a little amount of your fondant. Try to roll it out and taste it 
I don't think that the shortening would influence the taste. But I don't know exactly because the shortening I use doesn't have any flavour (in my opinion it doesn't taste greasy at all, there is no taste). I even make my buttercream with it - I use only a small amount of butter.
Luggi
Hey - I've been practicing covering a few cakes with MMF. I find that using powdered sugar doesn't work very well with MMF like it does with regular fondant. I do lightly grease my rolling pin and my work area and I haven't had any problems. If I'm rolling out enough to put on a cake, I do lightly dust with cornstarch and use it like I would powdered sugar. I haven't found that the Crisco affects the taste at all, so don't be afraid to use it. Just make sure you just lightly grease everything - you don't need a whole ton for it to work.
Hope that helps!
Melissa
How long will MMF last, and what is the best way to store? I just got my first ever cake order (just finished my last session 3 class last night!!!) she wants fondant and I thought I would do MMF since I have heard it tastes better, but want to do a practice run first and "play" with it. But dont want to waste it.
I don't let mine rest..I use it right after I'm done making it..so far I've had no problem with it, unless you are not ready to frost your cake..room temp is the best..no need to refrigerate 
there's no dairy product in it.
I've found greasing the matt and rolling pin works much better for MMF too. And the cake I just did for my dtr's bday did not have any issues with the other decorations sticking to it. I put a bit of peppermint extract in the fondant instead of water when I mixed it, so it tasted more like a candy cane anyway, but I've never noticed any taste being added to the fondant b/c of it. But, it's only a light coating to keep the fondant from sticking to the matt and pin.
I use powdered sugar when rolling the MMF. I don't add any shortening at all. I also use mine at room temp., usually just after I've made it and I've never had any problems. I like to use it when it's fresh because it's still a little warm and very easy to roll. You can always put it in the microwave to warm it if it's been sitting for a while. That's just my prefrence. I've also heard some people using a MMF recipe with shortening in it, which I've never tried.
Karate.....I'm sure that the food police wouldn't approve of this time frame...but I made some in NOVEMBER...and have kept it airtight and used it off and on for flowers, and small things for the past 4 months!!! Tastes fine..and works fine. Go figure. I read somewhere that somebody said TWO WEEKS. LOL!
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