I would say the worst bathrooms belong to the bars on sixth st. Or any live music venue. The alley between sixth and seventh is full of pee and poop most of the time.
When cruising for streetwalkers, I occasionally stop by the bathroom at la Quinta(35 and 183) or holiday inn on middle fiskville. There's been times when I could not make it to a bathroom and had to take a crap near the HEB working lot on 41st or the public storage parking lot next to the chronicle.