I am a 26 year old man of moderate means, so when I travel it means I'm usually staying with friends or family. I have come across an absolute steadfast rule regarding couch-surfing.
1) If the people with whom you are staying are under 30, you are an asshole for not bringing your own towel.
2) Conversely, if your hosts are over 30, THEY are the assholes if they are not able to provide you with a clean towel.
Addenda to this rule are as follows:
1) If your host are under 30, but married, they should have a clean towel for you as weddings net linens in absurd volumes. (My sister-in-law would argue that said volumes only appear absurd to me as I am a dirty bachelor.)
2) If your hosts have an actual guest bedroom to provide you, as opposed to merely a couch, they can pony up for extra clean towels.
It is polite to bring your own soap and shampoo, but no one really notices if you use theres as long as you stay away from personal loofas and the like in the shower. Using another person's loofa is just fucking tacky and gross.