They don't really have to be. Almost all of the tubes get filed down to sit nicely against the other tubes or get reamed/filed down once they're brazed into the lugs.

You're also going to need access to an alignment table and somebody who knows what they're doing at some point to make sure that your frame is relatively straight and that the spacing is right. Also, there are some really expensive, specialized tools needed to face/chase the bb shell and face head tube that you're probably not going to want to invest in unless you're building a lot of bikes or have a pile of money. A well-equipped repair shop or friendly neighborhood framebuilder should be able to hook you up and do that stuff for you.