If you’ve had your fingers in someone’s vagina, or had someone come on your hands, then it’s a good idea to wash your hands with hot water and anti-bacterial soap before touching your eyes or anyone else’s genitals (individually-packaged anti-bacterial towlettes might be useful if you’re outdoors). If your skin is compromised in any way, if you want to avoid needing to leave the scene to wash your hands, if you’re going to be engaging in anal fisting or exposing yourself to any blood, or if you just want to be extra-safe, then try using disposable latex “examination” gloves (available at most drug stores).
If you’re not going to use gloves, then just as a hygiene matter washing your hands before putting them in someone is a good idea.
Of course, if you’ve just had your ungloved fingers in somebody’s ass, then you’ll want to be sure to clean your hands particularly thoroughly (especially under your fingernails) before putting your them in or near anyone’s mouth.