No, the scripts are not 100% accurate. I believe many of the scripts are purely written by fans. A PERFECT example of this, is in "The Deal" when Jerry is trying to buy something for Elaine's birthday and George suggests the bust of Nelson Rockerfeller, Jerry says "Too gubernatorial" where on the script the person wrote "Jerry: Too Goobenatonal (?)" where gubernatorial isn't a part of the fan's vocabulary.
Then on that same script, where they were wondering about the kiss goodnight it says
Jerry: Tough one. You're call.
Elaine: It's brug-wa (?)
Heh, the person who wrote the "script" had no idea what Elaine said either. Heck, I don't know what she said.