if you've never done it b4, cum on her tits...then if she likes that, move up to face. she may not like that sort of thing, so cumming on her face could be a bad experience lol!
I personally have no problem "sharing the wealth" but many women see this as demeaning and degrading (of course, granted, some women DO enjoy being demeaned and degraded) so it's always best to err on the side of caution and hold off on the jiz bath until you have a definite indication from her that she's into it. Just my 2 cents worth...