For me the most important factor in performing cunnilingus or not is actually whether the girl is freshly showered (so no bacteria- or more accurately not too many and hence easily handled by the immune system). As for STDs if you want to increase safety of reverse oral you can use dental dams.
But let’s take the possible STD risks of (unprotected) cunnilingus one by one:
Herpes and HPV: Even condoms cannot satisfactorily protect you against them during vaginal penetration sex. Plus both diseases can be transmitted through French kissing, OWO and OW. HPV vaccines can protect only against 4 strains (2 leading to warts and 2 leading to cancer) - and there exist more than 100 different HPV strains (and of course more than 2 that can cause cancer). So basically the question about risking with herpes and HPV should actually be whether you want to have any sexual contact with a WG (or actually any woman) – not whether you will perform cunnilingus to her.
Gonorrhoea and chlamydia: Their transmission is very unlikely through cunnilingus since the infected site is the cervix which cannot be reached during cunnilingus.
Syphilis: It can only be transmitted in the presence of symptoms, so you should be an idiot if you cannot recognise them and don’t stay away (actually run away for your life).
HIV: The transmission chances are mostly academic than practical (like if you are having significant amounts of blood in your mouth from e.g. a dental surgery and the girl is also having menstruation).
Hepatitis: Hepatitis A and B can be prevented by vaccines (everyone in Europe takes these vaccines during childhood).
In other words reverse oral is arguably the activity with the lowest risk factor during sex (bar protected cunnilingus). If you don't want to risk at all then pay a WG for striptease, massage, wanking (handjob/titjob/footjob/DIY) and COB.