Are any of you guys loyal to a certain brand of butter for making buttercream?  How much does it matter?

ANope!

I (and many others) prefer a European butter due to its higher butterfat content.  Quality matters.

AI only know English butter. What's European butter then?

I actually like a generic brand.  It gets softer than some of the others which then mixes up better.

AI use my local supermarket brand (Tesco). I love it and wouldn't use anything else.

Tessie I have to buy what the small town grocery store sells.

I have used several different brands and never could notice a difference!

AI like the European kind

I tried a European cultured butter at $3.85 USD for 1/2 pound (8 oz).  It was 80% fat, just as the butter I get from Sam's Club.  I could not taste a discernible difference in the cultured butter in my SMBC.  

AEither Ralphs or Stater Bros. salted table butter. If I have time, I let it come up to room temperature. I don't always have time. (So far, the only thing I use unsalted butter for is when I make poultry dressing: I saute the celery and onions in one stick of salted margarine and one stick of unsalted butter.)