You can also use non-microwavable cling film.
The small holes that appear in microwave cling film (saran wrap, I think is what Americans call it) only do so at temperatures far higher than body heat, so it's usable too.
In all the global studies concerning HIV and AIDS, there are no known cases of HIV transmission through RO or through OWO for that matter.
To be fair, while I think that is right regarding RO, there are a few possible ones for OWO almost invariably involving CIM, but the risk of that - if they are HIV+ - is at most 'almost nil'. Very very little of the oral sex that goes on between men in the UK involves condoms, for example, and there isn't a link between that and whether they have HIV. (In contrast, there are studies that report that none of the men they looked at who have never done BB have HIV, while all the ones with HIV have, for example)
My cynical view is that dental dams were pushed within the lesbian communities as a way of avoiding talking about the real HIV risks there: sex with men and IV drug use.
For other STIs, the risks are also nil to very low - the arguments about whether some of the bacterial ones are ever transmitted orally to women still go on - but for some people, that's too high, hence..
With all due respect I think Amy has answered that question fully, the risk of catching anything via RO is minimal, but if you are uncomfortable doing it unprotected use dams or tell the clients you don't do RO. You should never do anything you are uncomfortable with.
.. is absolutely right.