AI've made a monster batch of shortening buttercream for cupcakes I'll frost in Saturday. It's in the fridge in an airtight container. Will I need to re whip it before piping or can I out it straight in the piping bag?

I'm testing a few by freezing them frosted now. Could I put the purchased icing flowers on too or would they bleed/dissolve?

You should rewhip and I would not put the decorations on if you are planning on freezing the cupcakes.

I usually freeze my monster batches of buttercream.  I don't really re-whip, but I do stir it with a spatula to get some of the air bubbles out and make it smoother for piping.  I've never frozen cupcakes with the frosting on.  

AI also don't re-whip, but like FrostedMoon I let it come to room temperature and stir well. I have froze frosted cupcakes many, many times. Always come out fine. I've froze up to a month.... A-Ok.

I agree, it's not necessary to re-whip buttercream. 

Stirring *WELL!* in small batches every 30 or so minutes in very necessary to get/keep out the air bubbles that develop.

You guys are right,,,,I said rewhip but did not mean again in the blender.  It is like mimifix said I stir real well with my spatula.  :)