I recently tried Italian Meringue Buttercream and love it as opposed to my all butter american buttercream. It is light and fluffy and not too sweet.

Now my question is i do a ton of tastings for weddings and usually I would freeze my leftover icing in souffle cups and just take it out a few hours before the tasting and give the brides a tasting box where they can mix and match, no rewhipping required on my end  but that was with american buttercream. Can i do that with italian meringue (it always says to rewhip) or will it taste off if I do not rewhip it?

I freeze my imbc all the time. I usually rewhip it before I use it. Although when I freeze cakes covered or filled with imbc I obviously do not rewhip. I have never felt any difference between the rewhipped and not rewhipped buttercream. That said, I have never tried the imbc by itself without rewhipping to see if there are any minor differences. 

I actually just made a cake yesterday with IMBC that had been in the fridge for two days.  When removed from the fridge it was as hard as a stick of cold butter.  After a couple of hours sitting on the counter it was still pretty stiff and in no way spreadable.  It still tasted delicious in the harder state, but I don't think I would want to serve it like that at a tasting.  It only took 10 minutes or so in the stand mixer to get it to come back to normal consistency.

Maybe do a little test to see how long it would take for a small container to warm up to room temp and if doing so will still give you the same consistency as if you had re-whipped it.

When you pull it out of the freezer I would let it defrost at room temp and then microwave for just a few seconds until the very edges are looking a tad bit melted, mix it with a mini whisk or spoon until well combined and you should get it at the consistency it would be when you apply it to a cake.

You will quickly work out he best time for the microwave if you are using the same size samples all the time.

Doing this will not only achieve the 'melt in the mouth' texture you are after but also the consistency that is best for the client to apply the icing to their cake when they are mix/matching icing to cake. I think using it without remixing/whipping is running the risk of the client gouging holes in the cake if the icing is a little chunky still when they use it.