To the person who said 51 or Pitu are the ONLY cachaça's to use, well -- this is simply wrong.
1st - Both of these brands can give you a nasty hangover and headache if you have more than one or two. It is true they are the most common brands. Think the difference between well and shelf liquor and you get the idea.
2. -- It is perfectly acceptable and fairly common for Brazilians to order caipirinhas with a better cachaça. I prefer Germana, from Minas Gerais where all the best cachaças come from, but I also like Triunfo (from Pernambuco, where I currently live). Ypióca, from Ceara, is okay if you get their 'gold' version but it's not fantastic.
It is true that there are a great deal of artisinal cachaças that are intended for sipping like a good bourbon, and are not meant to be mixed into a caipirinha. Particularly the darker blends. But to say that you should only use 51 or Pitu is not only very inaccurate, it's also cutting you off from a whole dimension of subtleties available with this fine drink.