It definitely depends on the woman in question whether she's going to appreciate the compliment of having a random stranger ask her out or just be annoyed. Odds of a "yes" are going to go up if it is someone who actually knows you -- assuming you're a presentable individual who's getting an acceptable score on her personal rating system.

Mostly the rule of thumb I think is to make your approach in a way that makes the approachee's day brighter, rather than one that sends her away fuming. Pestering someone who's clearly visibly busy or rushing to get somewhere is going to go down a lot more badly than speaking to someone who's sitting there peoplewatching or walking down the street in no apparent hurry. And you have to be watching for the hint if she isn't keen on talking and tries to shut you down or send you off.