If it's HIV you're worried about (and in my experience that's what most people are worried about) you need to remember first and foremost how it's transmitted. To be transmitted you need:
1) The virus to be present;
2) A fluid that can transmit the virus;
3) A means for the virus to enter a body.
HIV can be transmitted orally through blood, semen (cum), preseminal fluid (precum), human breast milk, cerebrospinal fluid (the stuff they take out in a spinal tap) and vaginal secretions. That means that if you are giving head to someone the virus can be present if that person has HIV. Of course, it is highly unlikely that you would ever notice cerebrospinal fluid during a sex act; however, cum, precum and vaginal secretions are common...blood less so, but it can end up there if things go wrong or if you're eating out a woman during her period. The viral load is too low in saliva to incur transmission. That means it's unlikely that you can be infected with HIV if someone is performing oral sex on you - unless one of those other fluids is in the person's mouth.
But it's not enough that the virus is simply present. It has to have a gateway into the body. This can include gum infections, recent dental work, recent brushing or flossing, sores, cuts, bites, and so on. Remember that if you're the one whose genitals are getting gobbled you have to watch out for these things on your bits and pieces.
The safest thing is to use with a penis is a condom. If you don't like the taste try putting it on the guy, spitting on it and masturbating him for a while. It'll taste more like your spit and hand and less like a lubed-up condom.
If you can't use a condom for whatever reason there are other ways to reduce risk. If you're worried, try to keep things to masturbation. Closely inspect the person's genitals for sores or cuts. Do not take semen in your mouth or swallow it. Do not brush or floss for a few hours before or afterward. Avoid oral if you have sores in your mouth or a throat infection.
It's not good enough to think of a sex act as high or low risk. You have to know how HIV is transmitted and realize that some ways of giving head (ie. swallowing) are a hell of a lot more risky than others (ie. using a condom).