The safest bet is to avoid any and all skin-skin contact where mucous membranes are involved (I.e. Genitals, mouth, etc). While typical oral has a statistically negligible risk for things such as HIV, the risk is increased for herpes, HPV/warts, etc or even an oral yeast infection. You are correct about your stomach acid being a good defence (hydrochloric acid), but there are less protected mucous membranes on your way to the stomach (like your mouth!). Brushing your teeth immediately before a situation where your mouth may be exposed to pathogens is one of the worst things you can do as the bristles on your toothbrush create tiny tears in your gums and other surfaces.
Most responsible SPs will be tested regularly, and never provide any 'bare' services (ie BBBJ). My concern wouldn't be with the cleanliness of those SPs, but what her clients previous to you may or may not have.
Use barriers.