I’ve used Helvetica quite a few times, so I believe it is, but I do always have Arial, ‘Sans-serif’ incase the user does not have Helvetica installed. But good question
Helvetica is commercial.
So you can’t have it in the zip the buyer downloads, but only in your font stack, for users who have it installed already.
Yea Just don’t include the actual font, such as doing it in @font-face. Here’s a nice list of web safe fonts: http://www.speaking-in-styles.com/web-typography/Web-Safe-Fonts/
Although there’s only a select few that are 100% web safe. Times New Roman, Arial, Georgia, etc. But there are quite a few you can add into the primary of your font stack in case the user has the font.
I’ve been checking around for free @font-face fonts and there’s quite a few good ones. Check out http://www.fontsquirrel.com/